That said, I will add my two cents and point out that most sincere practitioners don't believe in Christianity because it comforts them, helps them explain the world, alleviates their pain or loneliness, etc. That's just atheists projecting. Because the idea that other people believe in a faith because they came to the independent, logical, conclusion it was true terrifies them.
I'm going to disagree with you a little on this. Undoubtedly there are atheists who are condescending and dismissive of anyone with faith (we have many threads on such individuals and groups!). But I don't believe that the majority of any faith have truly and legitimately reasoned it out for themselves. Many have maybe. But many are just being as dismissive of alternative views as those shitty atheists and only sticking with it because the familiar is always comfortable. I've known plenty of those types and I can't believe you haven't.
I'm an atheist, but I don't doubt you when you say you've reasoned your way into a legitimate, intellectually honest faith in your religion. I also am not terrified or in denial that you are right and I am wrong. Why would any atheist take the path they do if they were so confident it was the wrong one? I've done a lot of soul-searching on my place in the universe and if it had led me to believe the Christian god was undoubtedle and the bible absolutely correct, why wouldn't I follow it?
I don't think reason can without a doubt lead a person to the one objective truth of reality, at least not in the short time any one person has to live. I've come to the conclusion that we are not capable of reasoning that efficiently, and that to believe we are may be insulting to that god if he is real. (So if anything I'm more akin to those Christians who say "I don't know and can't prove, but my faith leads me to believe taking a chance on this higher thing is the best choice").
Okay, I get the original meaning is "to stray from the mark" -- like literally an arrow not hitting the target on point. But then... what's straying from the mark besides it's not doing the original function of reproduction? I also get the concept that sex is first a reproductive act before all things. But then the question becomes: is that all it can be used for? Like seriously, using sex for just having reproduction is absolutely stupid, as most of the times it feels fucking good and that's the main reason why people do it. If God made sex pleasurable, the only time he would be calling it a sin is overtly abusing it-- and the definition of "abusing sex" is left for mankind to interpret, because that is complicated stuff based on your ethics and "morality". However, I think the more people realize that sex is just sex and nothing more, the more they'll realize that that is probably how "God" sees it; just a form of pleasure that, when not ABUSED, leads to some good times.
I mean, I think Oscar Wilde said it best: "Everything in the world is about sex except sex. Sex is about power.” is a good quote to run by for a second. Sex is powerful, but fun; just use it wisely.
Tl;dr the purpose of sex just to have children is just too narrow and is not absolutely correct, as sex feels good and getting a child out of your vagina is not so much.
I really have a problem with the idea that sex is only for procreation because it is observably not true. The majority of animal species on earth do mate only to reproduce, but there are many who practice mating behavior for another reason - social bonding/pair-bonding. Many of those animals include the other apes/primates with whom we share so many other characteristics.
If you boil down any practice to one obvious core purpose, then why not the rest? If sex outside of procreative purposes is wrong then how about hugging or kissing? You shouldn't hug a person unless it's within the bounds of matrimony and as a direct overture to begin producing a child. How about food? Food's only purpose is to nourish you, so if you ever enjoy the taste or eat any other time than you prevent an immediate health concern or starvation you're doing it wrong. Why do anything that feels pleasurable if you can make an arbitrary argument that a more mechanical purpose to that act takes precedence?
Which leads to another question that you can ask about an omnipotent god regarding any subject really - if sex outside of reproduction is wrong, why did god give us the drive for it? We already know that chemical imbalances, medical problems, emotional difficulty, etc. can kill a human's sex drive. We also know a lot of those animals I mentioned don't have their sex drives running all the time. So why did god set ours to always run if it's such a bad thing?