Heyyy! ^^
Practically, I think it'd be simplest to give a straight player both options (like kissing vs not kissing, or other mild sexual actions) without progressing any les. romance paths. That'd probably require a minimal amount of alternate flavour text.
Otherwise, you might consider giving the player the choice to decide how flexible/averse their MCs are to these actions as an additional Char-customisation choice on-top of their chosen sexuality. Then cross out or block the relevant choices or scenes that accord with their flexibility.
Just some loose shower ideas haha