The problem with M, I think, is very simple:
She's a very, very generic relationship. She's your age, there's nothing special about her despite being a single mom, she's an...average person. It's a porn game, there's a reason most of them are fantasy or incest or whatever. Even the most generic ones target something specific, the sweet innocence of young love or whatever. With M it's just an adult relationship - two people like each other, they end up having a date, having sex, M blows you, then there's some more sexy time. There's nothing spicy, nothing new. I'm not at all actually saying that is bad - I very much like the story. If anything, it adds a shitload of realism that many games miss, cause it actually feels real. But it does make M feel like a sidequest towards the main, spicy idea of doing her daughter.