You guys also reminded me of the ‘association’ between the sex position and the context of the moment:
Missionary: ‘First Time’ experiences for one/both character, ‘romantic’ moments, and occasionally guys being a ‘two pump chump’. The enthusiasm of the woman is gauged by her grabbing/scratching the guy or wrapping her legs around him. If she’s not doing these things, she’s either a neutral participant or more passively going along with it (In Rome, Lucius Vorenus initially having sex with his wife while she just kind of lays there and grimaces.
Cowgirl: Tends to happen when sex is happening with prostitutes (Fargo) or just overall slutty/uninhibited women. But in general, this position seems to imply a woman that does this in a movie is a whore, either figuratively or literally. Or dangerous/conniving, to illustrate how ‘in control’ she is of the guy.
Doggy Style: Popular in rape scenes or where the woman is in some variously unwilling state. Often used to show how ‘dominating’ the guy is over the woman. Also seems the most common sex position for gay sex scenes (for obvious reasons, but hey, I’m sure gay guys have sex in other positions too, they just don’t seem to show it on TV/movies much).
Other sex positions: Generally reserved for ‘crazy’ couples ([IChoke*, or as part of a joke (Team America. I don’t see many/any sex scenes with couples that are lying sideways and spooning, though in real life it is a popular position among people I know (intimate, comfortable, I don’t have to do any of the work 
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