Having gone through this game, I really have to agree that the story could use work. I am a fan of NTR, so this is not about me bashing the genre at all. I just think it would mean a whole lot more IF the characters were well-developed.
1) Hiroki is obviously going to be humiliated, but maybe he could have more positive traits, if only for the reader to 'feel' for the guy. Right now, we don't respect the main character at all.
2) The girl was a biatch from the beginning. Instead, have her start from being a loyal girlfriend, and slowly move towards corruption, as other people in the thread have already said. I can perhaps expand on this a little later in the day or something.
3) We don't know how good the black guy is at sex because the corruption never really happened - the girl was not a good character to start off with. Instead, we should see a difference from the 'vanilla' sex to the mindblowing sex that she should feel guilty about.
I am being pretty quick with this review, mostly because I don't have all the points in my mind. At the same time, I don't want to overwhelm the author, who I have to say is very good at taking criticism/opposing viewpoints. I don't necessarily hate the project, but I think the characters were basically an afterthought when you wanted to do the interracial sex.
But yeah, I hope you keep making games. The fact that it's your first game is really good. And as people have already pointed out, the fact that you finished the game is good. You should build on this, absolutely. Don't let any of the negative people tell you that you suck. You don't.