It seems that sex is always a sin, as evidenced by the fact that Jesus had to be born of a virgin and not by traditional conception in order to be sinless. Is this true?
Any viewpoint is welcome.
No no, it's not a sin. It could easily become a sin if we pervert that which was intended for good.
I believe that Mary's virgin Birth was a testification to the Truth of Christ. Could you imagine what the Catholic church would have done with the Husband? "Father of God" I'm sure that would have been completely chaotic. Who's really the father of God?
Well, we know through the virgin birth that only God the Father is the Father of Jesus Christ.
The first spoken words of God to humans in the Bible is the command to have sex:
"Be fruitful and increase in number; fill the earth..."
So it cannot possibly be a sin.
(Also, as Nicky Gumbel says on the Alpha course, God is not looking down and thinking, "What ever are they up to?! I didn't intend that at all!")
Mary's virginity at the time of Jesus' birth has nothing to do with whether or not intercourse is sinful or not. It has everything to do with the uniqueness and divinity of Jesus. God incarnate could not be born in a normal way. He was both the Son of God and the Son of Man.
An interesting note is that Jeconiah's line (the last king) was cursed and prohibited from the throne. Joseph's lineage still traces from that line, but Mary's takes a different path around Jeconiah. Through Joseph, Jesus had title, but escaped the curse by not being biologically tied to him.
Joseph married Mary prior to her giving birth, but he had no union with her until Jesus' birth. This is important. Some teach that Joseph never had sexual union with Mary, but this is supported only by certain traditions and not by the Scriptures. The Scriptures do refer to Jesus' brothers and sisters and James and Jude as such by name. So, Joseph and Mary had intercourse just like any other couple would, and they had children by that union as well.
God created sexual union and even commanded it, as noted in Genesis 1:28. God also speaks of children as being a blessing from God.
So, Mary's virginity has to do with the uniqueness and divinity of Jesus--not anything sinful with biblical sexual union.
This address the second part of your question about Jesus being a born of a virgin in-particular.
Because Adam was the first man, and everyone has descended from him, and because he sinned against God, everyone in the world has 'guilt by association' in essence. Please forgive me if this is an over simplification of the doctrine of the original sin.
Mary, being a virgin, excluded Jesus from among the descendants of Adam and therefore not only did he not commit any sin while alive, but does not have Adam's original sin on his head either.
The fact he was born of a virgin is not an necessarily any kind of indication that sex is sinful.
I would argue that your thinking has gone awry because you infer that Jesus being born of a virgin means that sex is somehow sinful and results in sinful offspring. This is a flawed premise.
Romans 5:12 says "Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all people, because all sinned". Therefore, it is necessary that Jesus, the second Adam, is not born of the bloodline of the first sinful Adam.
All naturally born men are born into broken relationship with God and as a result of that broken relationship are subject to sin (subject as in a King's subject). As slaves to sin, bound in sin, we can't help but sin - we are inherently sinful. Which is not to say that we have entirely lost the image of God (I don't hold to the idea of total-depravity), but rather that the image has been damaged beyond any human ability to repair.
Jesus, however, was not naturally born, but was conceived of the Holy Spirit, free from a sin-nature, and therefore free to be perfectly obedient to the Father. It was only in that perfect obedience that he was able to pay the price for our sin where we could not.
When sex is expressed entirely free of lust, as a self-giving love for one's spouse, it is not only right and good, it is actually giving glory to God because it is imaging the love of the trinity. Furthermore, it is imaging God's love in it's creative and live-giving power when offspring is produced. Therefore, sexual relations are not in anyway inherently evil, but like anything God created good can be twisted by man's sinful nature to become imperfect (to varying degrees).
(As something of an aside, I have often wondered if sin-nature is passed on from one's father. Such scriptural ideas as "the sins of the father shall be visited upon the 3rd and 4th generation", yet the contrary idea that no man is held accountable for another's sins, make me wonder if the way of the sins being passed on is via sin-nature, with a particular affinity for sins which afflicted your forebears. This, however, seems to be a modern idea, and one which theologians before the 20th century do not raise.)
We can take an idea in Mosaic Law. There we find statutes concerning uncleanness, and not sin, ascribed to seminal emissions, menstruation, and childbirth. (Leviticus 12:1-6; 15:16-24).
Leviticus 12:1-6 NIV
1 The LORD said to Moses, 2 “Say to the Israelites: ‘A woman who becomes pregnant and gives birth to a son will be ceremonially unclean for seven days, just as she is unclean during her monthly period. 3 On the eighth day the boy is to be circumcised. 4 Then the woman must wait thirty-three days to be purified from her bleeding. She must not touch anything sacred or go to the sanctuary until the days of her purification are over. 5 If she gives birth to a daughter, for two weeks the woman will be unclean, as during her period. Then she must wait sixty-six days to be purified from her bleeding.
6 “‘When the days of her purification for a son or daughter are over, she is to bring to the priest at the entrance to the tent of meeting a year-old lamb for a burnt offering and a young pigeon or a dove for a sin offering.
Leviticus 15:16-24 (NIV)
16 “‘When a man has an emission of semen, he must bathe his whole body with water, and he will be unclean till evening. 17 Any clothing or leather that has semen on it must be washed with water, and it will be unclean till evening. 18 When a man has sexual relations with a woman and there is an emission of semen, both of them must bathe with water, and they will be unclean till evening.
19 “‘When a woman has her regular flow of blood, the impurity of her monthly period will last seven days, and anyone who touches her will be unclean till evening.
20 “‘Anything she lies on during her period will be unclean, and anything she sits on will be unclean. 21 Anyone who touches her bed will be unclean; they must wash their clothes and bathe with water, and they will be unclean till evening. 22 Anyone who touches anything she sits on will be unclean; they must wash their clothes and bathe with water, and they will be unclean till evening. 23 Whether it is the bed or anything she was sitting on, when anyone touches it, they will be unclean till evening.
24 “‘If a man has sexual relations with her and her monthly flow touches him, he will be unclean for seven days; any bed he lies on will be unclean.
These statuses reminded the Israelites not only of their hereditary sinfulness but also of the need for a ransom sacrifice to cover sins (in general) and restore human perfection. Of course, the animal sacrifices they offered did not accomplish that. (Hebrews 10:3, 4). The only way was Jesus.
Hebrews 10:3 (NIV)
3 But those sacrifices are an annual reminder of sins.
My wife and I are reading Theology of the Body for Beginners by Christopher West and it elaborates on the first part of Genesis in a way we don't quite understand, but I can at least explain it.
Before the fall Adam and Eve loved each other, therefore they probably would have made love (the Bible doesn't say that, but it doesn't say how long they were together in the garden either, it may have been only a few hours!)
The only difference is, them making love would be 100% free from lust because they did not know sin at that point in time. I'm not saying it would be passionless robotic utilitarian love making, but it would be free from 'the passions' which cause us to use each other in a less-than-respectful way. It's something we can't quite get our heads around because original sin muddied the waters.
But, to answer your question directly, no it's not any sort of sin or grave matter to love your spouse in the way God determined you to do so. This doesn't require biblical evidence other than "Male and Female He created them".
However, you said the 'right circumstances' but it's more nuanced than that.
So, the circumstances may be right, but the object and intention may be wrong and those also need to be considered when making any sort of judgement to the gravity of any particular act.
To answer your question no sex is not a sin as long as you follow the laws of God i.e get married first. If you look in the Bible, contrary to popular belief, Christ has brothers and sisters, born of Marry from Joseph - I know crazy right? So obviously she did not stay a virgin for the rest of her life.
Also look at all the times the Lord blesses someone through their seed, Abraham, and Isaac to name a few. Do you really think that the Lord would bless people in this manner if sex inside the bonds of marriage were a bad thing?
As to original sin and whether it's passed from the father or mother - well according to the scriptures while we need Christ to atone for our sins (since we all sin since we live in a fallen state) we are only held responsible for our own sins and not for the sins of our parents going back to Adam.
2 Chronicles 25:4 (KJV)
"...but did as it is written in the law in the book of Moses,where the Lord commanded, saying, the fathers shall not die for the children, neither shall the children die for the fathers, but every man shall die for his own sin,"
Ezekiel 18:20 (KJV)
"The soul that sinneth, it shall die. The son shall not bear the iniquity of the father, neither shall the father bear the iniquity of the son..."
Galatians 6:7 (KJV)
"Be not deceived; God is not mocked: for whatsoever a man soweth that shall he reap."
So we are all responsible for ourselves and what we do, not for what our fathers did or what Adam did (that's why Christ came to earth to free us from our sins because we can sin ever since Adam committed the first sin, removing man from the presence of God).