Obi-Wan is a naturally friendly and personable guy who just happens to have sex with, like, everyone he meets. It's pretty wholesome and aboveboard and it takes Anakin a while to NOTICE, because Obi-Wan is discreet by nature and Anakin is first a baby, later slow. But eventually it kind of starts to. obsess him? He wouldn't mind so much if Obi-Wan were just in a committed sex-no-love relationship with someone! He thinks! Buuuuuuuuut...

anyway you can take this in any direction but bonus bonus bonus if at some point Obi-Wan has a relatively unsatisfactory encounter, comes home, and Anakin fucks the other guy's spunk out of him while he's still sore and loopy :3