Like Feierflei said, there's no connection between Sunville and YU. The cameo is just a fun little nod for fans of both games, and also a small token of my gratitude for the help Feierflei's given me.
YU is
broadly set in the real world. If you'd like a story version for the lack of men met/seen, our player character is just much more interested in meeting women right now, so doesn't really notice guys around. I haven't explicitly said the uni is an all-girls institution, and I think it makes more sense story-wise that it isn't.
If you want the behind the scenes reason:
I just don't wanna design and add in a load of dudes