i think about this all the time!! i have many different scenarios for how they started getting kinky 🤔 my favorite one is that they discovered their kinks at the same time as they experimented together (because theyre each others firsts), so there was no specific "introducing" moment. wendy feels anxious about not being in control - it affects how she prefers to have sex with stan. they discuss it together (as in, what she wants them to do this time. or maybe stan suggests it first 🤔), she discovers bossing him around really turns her on, and stan discovers letting wendy take over turns him on too. it just happens, really.
but thats mainly for their sub/dom dynamics. i hc both wendy and stan with some taboo kinks too. for wendy's incest kink, i think she would feel too nervous about it to sit stan down and explain it to him. she would test it out in the act, just as many other things. even if wendys sudden "call me your mommy" takes stan off guard at first, hes in the middle of having a good time and too horny to think about it too hard, so he just does. he might bring it up later, when theyre lying next to each other in bed. wendy would avoid elaborating "im tired, just go to sleep stan" or would shyly answer his questions.. i think her strong personality and past trauma makes her scared of being too vulnerable and its a rare sight to see her not being confident, just awkward in her wording. stan finds it cute but is always gentle with her in those moments.