Actually, those who talk about the age gap don't take into account how immature Midori is/has been depicted as being.
He clearly doesn't have a lot of experience. He's been together with one, count one, girl, and they clearly haven't been together all that much, since she's been super focused on her career, and he's supportive of that (and has focused on his own career (becoming a vet isn't one of the easiest courses of education) instead of hanging around Momo).
So he's avoided thinking a lot about relationships, or being a lot with Momo.
In contrast, Sui has been thinking about those things a lot, so (apart from her total inability to see that Ayame is behaving in a very unnatural way) she's about as mature as Midori.
Young males tend to be slower to develop in general ...
Sure, 7 years is a lot, but with the time jump, she's had time to catch up, and the older they get, the more equal they will get.
And like I said, it's not like he's a super experienced womanizer. He's pretty pure and innocent.
