It only works, because it's hentai. Young mothers have been done before, but there's limit to how young they can be and be at all believable. Looking at that woman, it's hard to believe that she's more than a few years older than him, if that. There's no way that she's old enough to be his mother. But it's hentai, and they were going for the joke - and it _is_ pretty funny, but it would have been _much_ more believable if they'd said that she was his sister.
The MC is sweet but very naive. I mean, why do women think that men they 'win' or 'steal' away from other women will somehow be faithful to them? I know this wasn't the case here, but she didn't know that until the end. Upto that, the guy she loved seemed to be cheater willing to screw over his fiance.
Ah yes, sweet, Vanilla filled misunderstandings! Misunderstandings when it comes to love can lead to such great things.
Great ending though.
His...mom, eh? I have to say, I was kind of hoping she'd get some action too, but this was nice enough as it is.