Assuming you mean Christians and not Freepers: You do realize that being a Christian doesn't mean one is sinless. It means we sin just like the next person; sometimes more so. When one is a Christian our hearts repent for our sins and they are forgiven by Christ so that we can be righteous in God's Holy presence.
I doubt you'd get many clamoring to answer that! But there are many freepers who condone pre-marital sex and "live-in relationships" -- they are a product of this society.
Speaking for myself, no I haven't. Would I have done this when in my early 20s I was agnostic at best? Probably yes, but didn't get the opportunity -- at times the opportunity just slipped away and at other times I "didn't get it" -- now I believe it was God giving me a helping hand at a time I was weak. Then, I just thought I was a nerd!
Since I've been married I have no such intentions -- not because I'm "better than" most people -- I'd say I'm about average in getting tempted, but thanks to God's grace I don't act on it -- I really do honestly, deep down, believe that marriage is a Sacrament, a duty, a Holy call, a vocation. To me to cheat on my spouse is more than just cheating on her, it is sinning against God by rejecting Him.
I think the problem in our society stems from us accepting this change in morality from "marriage is a Holy Sacrament" to "it's just sex, preventable or gay or whatever"
Imho, the way in which we have devalued marriage has made "gay marriage" acceptable -- if it's just a contract, big deal, eh?