Its a bit iritating to see straight dood's suddenly get this weird enjoyment in getting staffed by futas/hermaphrodites, so I propose this:
-long ago before the futas were futas they lived on a planet where there were both females & male, but tragic event (you can decide what kind) ended up with only females being the only gender left,
-thus trying to continue their existence they ended up creating futas as additional new gender, or initially the females had no other choice but to turn them self in to futas (you decide), which led to being either half/half female/futas, or little to no females and majority futas, or just ended up with all females just turning to futas entirely,
-that way you have in here main reason why your story initially showing only futas by them selfs,
-than the son & mom taken on the ship to be used to show initial long forgotten difference betwean both oryginal genders and how initialy both interacted with each other when it came to reproduction,
-once again instead make it another bulshit about futa to male thing you can make instead where it is made to where the son is the one stuffing the females and futas instead, which leads to all the futas fall deaply i love with the son including his mom and any other females that still were left,
I hope this will help in some way with this game creation and I look forward in seeing how you (the creator of the game) will progress the initial story.