It's influenced partly by the standard narrative that sex is supposed to mean something; that its sort of an eventual end goal that you reach once you've done all the other relationship-y type stuff so you've got some sort of connection with the other person way before your genitals connect. It's also influenced by approval-seeking mental business that I wish I could shut up.
This sometimes vaguely affects people like Now-hubby, Frisbee, or The German. I'm guilty of lack of confidence and security within those relationships, but I can typically get over it with some reassurance that what we have is significant and important in some way. This high school, "do they like me?" business applies more to people I've only just met, or to people I've had sex with before establishing any other type of connection (i.e. guys at the gang bang). Say...I've fucked somebody at the gang bang and then later during a break we get in a discussion about Rufus Wainwright or Doctor Who or any of those usual things that on a "normal" date I'd be all..."wow, this sort of makes you a cool person."
And then on a normal date I'd like to touch genitals with that cool person. But in the context of casual sex we've already touched genitals, so there's no established train of events because we've already skipped a few stops. There's no such thing as transparent emotions, either. Is it bad form after sex to ask, "what do I mean to you?"