So I haven't looked at this game itself, but I have seen the art that the person who drew this does and trust me, the protagonist changing form as they get corrupted is normal for them. Usually there is some outside agent (i.e. monster, drugs, curses... oh wait that's here), but as you can tell in the initial image all of those characters are probably the protagonist in various states of corruption, going from pale skinned to darker as they get more corrupt and turn into a hermaphrodite.
There's a series they do where it's a group of sentai rangers and the blue is turned into a monster by a monster king, then proceeds toa transform the rest of the rangers by having sex with them, including absorbing the pink ranger.
GFF is interesting to say the least, no idea about this game in particular.