This is a topic very close to me at the moment. My current girlfriend comes from a religious background and had some very, very conservative views on sex.
Whatever the bible might say, current interpretations of Christianity, which is what I think we're all talking about here, seems to put a dark mark on sex. It's not to be enjoyed, only to be used on procreation. Fun verses from the good book will tell you that it's a sin to copulate out of wedlock, a sin to waste your seed, a sin to cheat, and a sin to be homosexual. So yes, it would seem that this religion views most sexual acts as sinful. Why, I can't tell you.
It probably has something to do with the immense emotional weight of sex. It's as close as you can get to a person- literally and, in my opinion, spiritually. Personally I think it's something not to be done until you know both sides are ready for it. But a 'sin'? Not at all.
The real sin is repressing your thoughts, feelings and desires until they rot inside you. Just ask a Catholic Priest.