On the contrary, sex is what you do for the fun and pleasure the of it. The relationship comes later, if at all. Every sex partner doesn't have to be marriage material. In fact, few are. 
I'm not saying it has to marriage material...but I wouldn't equate sex with anything other than dual love. AKA: Love from and for both parties.
I don't have sex for the fun of it. I don't have sex because I'm bored. When I have sex, it's making love. It's not a fuck. It's not a one night stand. It's making love. It's being with someone I love, someone I'm in a comitted relationship with, someone that I know, as far more than a friend. The relationship comes long before. A year, if not longer, before, actually. I can have fun, making love, but I can't have fun, having sex. I don't have sex, just for the pleasure. I can get pleasure, in making love, but I'm not going to have a fuck, just b/c I need to get my rocks off. I dont need sex. Sex isn't a necessity to live.
-------
*Forever & Always...Yours*