I don't really know where the religious connotations came from, but sex is something that I would consider special in that it's the physical bonding of two people who care deeply for one another. That, as has been said, is making love.
Then you have plain old sex where you and someone who are both physically attracted think hey I've got a great idea, but it's just for fun and there's nothing deeper involved. That's simply sex, and it is driven by lust.
As someone once said, love and lust are not the same thing, but if you simply love someone and don't want to be with them physically, that's not the same either.
If I were to have sex with my significant other(s), I would want us to truly enjoy it and make the most out of it. Why bother doing it if you aren't going to put your all into it? Regardless of the circumstances, you shouldn't go into sex half-cocked, no pun intended.
Would I be willing to have sex with a girl who I didn't know all that well? Sure, if it was important enough for her to ask me. It wouldn't be the same (see lust again), but I'd certainly try to enjoy it and make sure she did in return.
So yeah.. is sex something "sacred"? I dunno what you mean by that, but is it something that should be respected? Damn straight.