The Whole Law is fulfilled in One Commandment but they still promote religious traditions and call homosexuality a sin. There have been wars in Christianity, judgments, persecutions.
How many Christians and from which section, do they believe there are no sins except not following this:
"For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself." - Galatians 5:14
Jesus said: “Love the Lord your God with all your heart, with all your soul, and with all your mind. This is the greatest and most important commandment. The second is like it: Love your neighbour as yourself. All the Law and the Prophets depend on these two commandments” (Matthew 22:37–40). We will never love our neighbour as ourselves if we do not first love God with all our heart, mind, and soul. In order to live according to the greatest and most important commandment, Christians must OBEY God’s laws.
So when God condemns sexual immorality and men having sexual relations with men and women having sexual relations with women, then Christians are obliged to live according to God’s decrees. There is no point in trying to twist Scripture to condone sexual immorality or to suggest that loving your neighbour as yourself gives men permission to have sexual relations with other men or for women to have sexual relations with other women. That is a perversion of the gospel.
I’ve been a Christian for over 20 years and although I am aware that some denominations condone homosexuality and sexually immoral behaviour, I have never come across any Christian who thinks that there are no sins except for the one you quote from Galatians 5:14. Anyway, verse 13 warns Christians against satisfying their sinful nature. When Paul says to serve one another in love he is not talking about sexual love. He is talking about “agape” love – the love that Jesus exemplified when he laid down his life to atone for our sins - selfless, sacrificial love. I fear you have taken this scripture entirely out of context. I found an article that explores what it means to love your neighbour as yourself. It concludes:
“Anyone who does not have a personal relationship with God through the Lord Jesus Christ will die in his sins and face eternity in hell. Therefore, we owe it to our neighbors to lovingly share with them the good news of the gospel. True believers have been forgiven, possess eternal life, and have blessings forever as the result of others who have shared the gospel with them. God’s love is evidenced in us as we communicate this precious gospel and love others as we have been loved.”
That's what it means to love your neighbour as yourself. But first, you must learn to love God with all your heart, with all your soul and with all your mind. And that means you do not disobey God or indulge the sinful human nature.
I see that this question has already accepted an answer, but the comments suggest that some concerns still remain, so I'll attempt to provide an answer that fills in these gaps.
I would think that most Christians agree that we should love our neighbor as ourselves. The disagreement concerns what is considered to be loving. This would to come down to a language issue. The English word "love" is often used in a variety of ways. However, the New Testament was written in Ancient Greek, which happens to have four words for "love", each concerning a different aspect. I'll briefly describe them here:
Jesus wept. So the Jews said, “See how he loved [phileo] him!” (John 11:35,36)
You shall love [agape] the Lord your God with all your heart and with all your soul and with all your mind. This is the great and first commandment. And a second is like it: You shall love [agape] your neighbor as yourself. On these two commandments depend all the Law and the Prophets. (Matthew 22:37-40)
For the whole law is fulfilled in one word: “You shall love [agape] your neighbor as yourself.” (Galatians 5:14)
Beloved [agape], let us love [agape] one another, for love [agape] is from God, and whoever loves [agape] has been born of God and knows God. Anyone who does not love [agape] does not know God, because God is love [agape]. (1 John 4:7-8)
From this, we can see that when God asks us to love each other, He is referring to kind acts; nothing sexual or romantic is implied. But this still leaves open the question of why homosexuality goes against this and is considered a sin. This can be answered using a couple of passages:
But from the beginning of creation, ‘God made them male and female.’ ‘Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh.’ So they are no longer two but one flesh. What therefore God has joined together, let not man separate. (Mark 10:6-9)
“Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh.” This mystery is profound, and I am saying that it refers to Christ and the church. (Ephesians 5:31-32)
When God created marriage, He made it as a union between a man and a woman. This relationship is designed to be a reflection of Jesus' relationship the church (the church being a term the Bible uses to refer collectively to all believers). We must remember that it was God who created marriage and not us. Any deviation from this form of marriage would go against God's design and would be an act of unkindness towards God, and therefore sinful.
It should also be noted how this relates to judging others. Let's consider what Jesus said regarding this:
“Judge not, that you be not judged. For with the judgment you pronounce you will be judged, and with the measure you use it will be measured to you. Why do you see the speck that is in your brother’s eye, but do not notice the log that is in your own eye? Or how can you say to your brother, ‘Let me take the speck out of your eye,’ when there is the log in your own eye? You hypocrite, first take the log out of your own eye, and then you will see clearly to take the speck out of your brother’s eye. Do not give dogs what is holy, and do not throw your pearls before pigs, lest they trample them underfoot and turn to attack you. (Matthew 7:1-6)
God has asked us not to judge, but this does not mean that there is no difference between what's right and what's wrong. A speck is still a speck, and a log is still a log, and Jesus says both ought to be removed. There is still a difference between pearls and swine, and Jesus says that they don't mix.
To further clarify that homosexuality is considered a sin, the Bible make an unambiguous statement:
Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality, nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God. And such were some of you. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God. (1 Corinthian 6:9-11)
I'd like to conclude by bringing your attention to the last part of this passage: we all were born sinners. I will not deny the homosexual feelings that people have as being something that they have control over. All of us, every single person, is born with a sinfully nature that we cannot necessarily control and is opposing to God. But if we desire to follow Jesus, we must deny ourselves, take up our own crosses, and follow Him. God has promised that anyone who truly believes in Jesus will be set free from their sins. We may not become sinless all at once; it usually takes a lifetime. Ultimately, this is what the the church is: a group of sinners who have trusted in Jesus for their salvation. All of us are sinners, and we should not judge or considered others to be more sinful than we are, whether they are greedy or drunkards or homosexuals or anything else. We should be accepting of everyone and help each other the best we can to grow closer and closer to God.
