If you think about it, before a female has sex she really doesn't desire it like she does after, and to some extent this is true for males also. I mean don't get me wrong we do want it before our cherries were popped, but in retrospect wasn't the eagerness just nerves and peer pressure. Its not the same after for either party involved.
Maybe its not just the sex tho? I mean you have all heard about the one night stands that you never saw again and if you did you acted as if you had no idea of their existence on earth. So why don't we cling to them for dear life like we do our significant others, FWB, boyfriends/girlfriends etc. What is it about the parameters of a relationship that makes sex so much more potent. The obvious answers are that there is a lot more emotions and feelings involved and I understand that but its still the same act isn't it? It's still sex.