How can people invest their whole lives in A Book of Rules a.k.a the Bible?

I don't mean to be rude, but I keep reading these answers about how ity is a sin in the bible and thus is not okay. And how premarital is a sin in the bible. I'm just wondering how it is that you can devote yourself to a set of "rules" so strictly. Why not say "uality is not right, to me personally, because it seems unnatural", or whatever. Why does everyone just say, "Oh, well, it's in the Bible that its a sin, so its a sin". I don't mean to be rude, so please, don't take it that way. For Christians, maybe there are some of you who question living by the standards, at one time or another. Afterall, it is a human written book. Just because it has been around for so long doesn't mean it's true or right; though, I'm not saying it isn't.