Now this I have a problem with. If you give choices in the game, it needs to make at least some sense internally. Some amount of head canon and player interpretation is expected in these games, but you can't just ignore the problem entirely. Unless you close the other routes entirely when you go steady with a LI, the MC's behavior needs to be addressed. The MC can pursue and bang every female available, then tell Carol he loves her, and then go right back to banging some random chick at a club without any self reflection. Sure, so far, we the players could just imagine that he lied about loving her and he is just stringing her along. But that is already a huge stretch. We know the MC is generally a good guy, and as you say, there are no paths for him to be a real asshole. It just doesn't make sense for this character to be able to do this without any inner conflict. Plus, it's rather apparent that the MC will continue to act in a way as if he was indeed madly in love with Carol during her scenes if you stay on her route, so it's not really possible for us to ignore what we see on screen and just assume that he doesn't really love her when chasing other girls.
This problem is actually not that hard to address. It just needs some attention to detail. It's doesn't have to exponentially bloat the workload by creating a cheating route or something. You can keep everything the same, but check the variables and make some relevant extra dialogues at appropriate times. If you told Carol you loved her and then face the choice to go home with the club chick, there can be a thought along the lines of 'I love Carol... I think she is the one, but damnit, this chick is too hot.' He can go over his actions on his bed when he is thinking about what to do. If he is going for multiple woman, he can feel some conflict and feel bad about what his doing to Carol if the +rel is high. It just takes some variable checking and a bit of extra text to make things believable enough. At some point down the road, there can be a reckoning where the girls deal with the manwhore MC, either kicking him off their route or finding some other resolution depending on their personality.