Huh, Traveller 5 sex
Well first there's the UPP extension Gender, Gender Preference, Body expressed as XXX.* Then there's the [C3] (Endurance) based Sex skill with its various knowledges, oral, vaginal, oral, and spawning. There is a a chart that cross references strategies and preferences to give a task target number on 2d6 that doesn't work with the task system in general. A three page statistical justification of that design decision. A variety of illustrations that look like they were drawn with a plastic template and a sharpie. And of course, ties to the genetics and sophont creation rules as well as two pages on the ethics of sex with alien animal encounters.
I love T5!
Just to be clear there are also alien alternatives spelling out SEX. Species and Euphoria. Body remains the same because everybody knows a sexy body when they see one.