It is important to understand what 'Sin' is and how the Early church lived and what is the orthodox teaching. First of all, sin in greek means 'missing the target'; Sin has no existence at all. The Church Fathers are consistent in this regard. Sin is a disease it is a sickness that affects all ...
"for unless we say the flesh was capable of receiving the things which belonged to it, with the exception of sin - for this is NOT part of the ousia, but a sickness which, as I have said, occurs as a result of inattention.." - St.Severus of Antioch
Because sin is nothing in essence and has no existence, it can NOT be part of our nature. Sin is only a sickness, it is missing the target (of course our target is Christ and his recapitulated life here on earth). Understanding this is important if one was to understand what sin means according the Fathers. Sin can not be past from parents to children, simply because it doesn't exist; it is not part of our nature. Man is by nature corruptible and mortal, not, man's nature is sinful (nature here of course meaning in the essence of Humanity). I could add in more quotes by other Church Fathers but i'll try to be as short as I can.
Another important point to make is that condition and nature are not the same. Man was always by nature mortal and corruptible because he came into being from nothing (St.Athanasius and St.Severus). But because God granted him a grace (to live in a CONDITION that is ABOVE his natural state and that transcends his natural state, he was able to share (through external participation (grace) NOT in essence with God's immortality and incorruptibility)). That does NOT mean man's nature changed .. No! Nature does not change. A chicken's nature that doesn't change into a horse nature, nor does a man's nature changes.But the condition changed .. Adam lived in a condition that was ABOVE is natural being, but that was only a grace given by God. Therefore when adam sinned, his nature did not changed (he was always mortal and corruptible) but his condition changed, ie. He went back to his original state and lived in that condition that is matching with his nature ...
"This, then, was the plight of men. God had not only made them out of nothing, but had also graciously bestowed on them His own life by the grace of the Word. Then, turning from eternal things to things corruptible, by counsel of the devil, they had become the cause of their own corruption in death; for, as I said before, though they were by nature subject to corruption, the grace of their union with the Word made them capable of escaping from the natural law, provided that they retained the beauty of innocence with which they were created." - St.Athanasius, on the Incarnation
"If however, he had continued to maintain his vision of God, he would have transcended the natural corruptibility and remained incorruptible" – St.Severus of Antioch
All this was just to introduce the idea that sin, has no essence, and is nothing. Thus I ask ..Can we move darkness?? ofcourse not .. why? because it does not exist (it is the lack of light!) but conversely I ask, can we move light? Yes we can, as light in essence is made up of photons, packets of energy, etc. Therefore as sin is nothing and can NOT be part of our nature as the Holy fathers of the early church taught us, then it can not be transmitted, let alone through sex.
Moreover, in Psalm 51, the following is read .. "and in sinS did my mother conceive me". This is the ORGINAL translation found in the septuagint LXX. It is clear that it is sins (plural) not sin (singular). The jewish Scholars knew what they were doing when they translated the hebrew into greek. David the prophet here is NOT talking about a specific form of sin that we are born with that has been passed onto us from our parents, but rather he is talking about the sinful experience that our parents are passing unto us and that we will pass unto our children. Since the Septuagint has sins and not sin, it clarifies what David the prophet meant; sins are endemic to humanity.
Last but not least, our church Fathers are very clear about how we are born ..
"The sin of those who engendered us, viz. the sin of Adam and Eve, is NOT NATURALLY MIXED WITH OUR SUBTANCE, as the evil and impious opinion of the Messalians ... we are born mortal insofar as we are born of mortal parents, but NOT sinners insight as we of sinful parents. For IT IS NOT TRUE that sin is a NATURE and THAT IT NATURALLY PASSED FROM PARENTS TO THEIR CHILDREN" - St.Severus of Antioch
WE inherit the condition and the consequence of the original sin, we are still born dead (separation from the source of life), but we are not born sinners, baptism is the remission of sins of course, but baptizing infants is allowing them to die WITH christ and rise anew, being the abode of the holy spirit; St. John Chrysostom teaches us ...
" For this reason we baptize even infants, though they are sinless, so that there may be given to them holiness, righteousness, adoption, inheritance, brotherhood with Christ, and that they may be his [Christ’s] members".
We are born mortal nor sinful, we baptize infants even though they are sinless in order to give them the trinity and the seal of the Holy Spirit (St.Gregory the Theologian).