He enjoys fucking you, feels your good qualities out weigh the bad... just enough to continue the relationship, but not enough to see you as marriage material. You're just Miss Right Now.
It's a very likely scenario if he's a high functioning sociopath. (It's estimated that 4% of the US population are sociopaths)
Source: High functioning sociopath and it describes how I've felt about most of my long term relationships. I don't want to marry them, and if they tried to force the issue, I'd probably leave. I've had that conversation twice.
Most sociopaths have their own strict moral code that generally lines up pretty closely with the moral code of the society around them. A sociopath generally doesn't get pleasure from hurting others (that's a sadist), the difference between a sociopath and the average person is that when an average person engages in a questionable act, they'll feel guilty about it, a sociopath won't. Mainly because a sociopath will generally have weighed out the pros/cons against their moral code before acting. This is why they're high functioning. A sociopath without a strong moral code of conduct and a willingness to stick to it, doesn't last long.
(My own situation, feel free to read or skip)
If my current GF brings up the marriage conversation I'd have it again. Fortunately, I told her early on that have no desire to get married & have kids, she still throws out hints about marriage occasionally... if she weren't set to inherit about 2 million when her grandfather dies (and another million when her uncle dies), I'd have ended it by now. She knows I'm a sociopath btw.
Her biggest flaw is at odds with how I want to spend my life... I like my living space spartan and clean, she's a specific type of hoarder who doesn't feel comfortable unless she is surrounded with mementos from every friend/relative and experience they ever had together... and that's not a joke. She refused to throw out any of her grandmother's stuff when she died (she was raised by her grandparents)... she couldn't even bear to let go of the broken cheap plastic "made in China" crap, just because it reminded her of her Grandma... never mind the fact that she literally has 1000 other things to remind her of her Grandma. But because that bit of broken plastic is tied to a specific memory she holds, it has the same emotional value to her as the family china that's been passed down.
Being a sociopath, every possession I own is filtered not by who gave it to me, but by its usefulness, its comfort, and whether it is in keeping with the general aesthetic of the room it will be kept in. I don't keep knick-knacks, regardless of who gave them to me.
2 million, It's enough to entertain the possibility of marriage. With $350,000 we can have a house built, she can have a wing dedicated to her shrines to the past, and I can have my spartan living space, and we can both retire early.