1. Ap. Paul not refer to lust but adultery.
It's clear from the context that St. Paul is talking about lust, not adultery. Adultery is something that would come after someone got married, whereas St. Paul was saying that this thing that he was talking about (avoiding lust) was a reason to get married in the first place. His thought is: it's good to remain celibate, but if it comes down to you choosing between 1) burning with lust or 2) getting married, then you should marry. And obviously it wouldn't help you avoid fornication (ie. lust) if you were married but not having sex: if anything, it would exacerbate the problem.
2. St. John , with all my respect have no sexual experience, and would not teach about nature of sexual intercourse publicly …. even then it could be his personal theological opinion.
It could be his personal opinion, except that, as I understand it, it's what the Church as a whole teaches. To quote another Church Father:
"Let no one think however that herein we depreciate marriage as an institution. We are well aware that it is not a stranger to God's blessing. But since the common instincts of mankind can plead sufficiently on its behalf, instincts which prompt by a spontaneous bias to take the high road of marriage for the procreation of children" - St. Gregory Nyssa, On Virginity, 8
St. Gregory here not only says that marriage is blessed by God and a "high road," but talks about how humans naturally ("common instincts") are attracted to one another. Since he goes on to talk about the procreation of children I believe we can infer that St. Gregory is not just talking about desire for companionship or love, but also sexual attraction.
Also, according to Orthodox epistemology one does not have to have direct experience of everything to understand the nature of man. One need not experience homosexuality to understand it, or the use of contraception, or what it's like to be someone of the opposite gender. Now, certainly experience counts for a lot, I am not arguing with that. However, when the tarnished image of God in someone is cleaned up, and they make great spiritual strides, they begin to understand the deeper mysteries (human nature, salvation, etc.) And I think that many people would agree that, of all the Church Fathers, St. John was one of the best at being pastorally sensitive and understanding humans as they really are in reality (not just in theory).
Let see what St. John speaking about in real:
What way one flesh occur?
Same way if you obtain most pure gold and mix it with other gold. So here we have some thing simular….. wife receive “melted like” fruit substance in hit pleasure moment, than feed it and keep it warm, give to it all needed and produce (new) man. And baby serve as some sort of bridge , so it is three combining in one flesh, for child connecting one side with other.
amour à trois ?? no way St. John say it.
No, St. John was talking about the child being a bridge between the two parents (and not a necessary one for marriage to have meaning, as St. John also says). He wasn't being literal, nor do I see how he could have been literal...?