As a practicing Catholic, no offense, but you're way off base. The point of making things like sex, gluttony, etc taboo is that it's a pleasure that you're putting above God that detracts from the worship of him. In many western societies people tend to think "victimless crimes" are of no consequence. I'd also like to point out that you seem to be working off the assumption that there is no God. If in fact you were operating under the assumption that there is in fact a God or deity, etc. simply this higher order being commanding it is enough to follow his word.

I would also say that on a personal (non-religious) note, I (and many others) consider sex a highly personal and special act that becomes less so the more people it's done with. Valuing modesty isn't such a bad thing, and I believe that being select about who you have sex with makes it more valuable of a gift to share with someone.