I do have more scenarios base in established pathing if you want. I'm glad to help has very few seem to actually use the terminology correctly and seeing a whole game made around different relationship dynamics really caught my attention.
some additional if more broad scenarios-
mutual birthday gift- mc and mc's gf want to have a baby but mc is shooting blanks so mc offer his gf to his younger brother on his birthday/ possibly adding options for mc giving younger brother sabotaged condoms, and wether or not said condoms are even used.
beach visit - mc and mc's gf encounter a surfer boy and his "gf" and they propose a swap (differing paths for the beach couple to be siblings or stepsiblings just looking to get laid but otherwise haven't lied/ or the "gf" being an effeminate younger boy/possibly younger brother of the surfer boy)
taming the nympho- mc's gf has an over active libido and goes through a lot of sex toys, mc needing help and can't be present 24/7 needs help to avoid her libido going out of control, and turns to a friend or trusted mutual roommate to help in case mc isn't available possibly with mc joining if walking in on them.
the following ideas really depend on how much you want the game to be anchored in reality over fanservice and fantasy
a friend in need- mc has a friend (Carlos or other) that has an medical condition that requires frequent sex and the mc offers his gf (would probably feature a lot of condom content.)
(can't think of a fitting name at the moment) - both mc and mc's gf hate humanity and being human but also live in a world with anthro's, anthro roommate and his anthro gf swap ( either with hybrids being the result or one way breeding and consensual cucking to an extent mc's gf is impregnated by the roommate with mc's knowledge and blessing while mc has sex with the roommates gf and can't impregnate her.)
I have a few more ideas but again kind of want to know how realistic I have to be for them to be helpful here.
I seriously loved the demo and hope to see more.