At the moment I'm pretty sure all the endings for the game are done, which I've done them all, so I'll tell you things that will happen without spoiling much.
1) Yes, Gomamon is the only one you can have sex with.
2) Drimogemon is an optional non-con encounter, if I remember correctly. (Though there is an optional encounter right before the first real 'hurdle' of the game
(optional in the sense of if you want to view it or not, that will happen in canon no matter what you pick. It's for plot reasons I guess, lol.))
3) It's not possible, but you can use his genital slit like a pussy.