That's something I've often wondered myself. There's a peculiar lack of femdom titles where the PC isn't a skinny twink-looking kid. Masculine men, or hell, men in general being targeted for those acts are borderline unheard of. I don't get it at all, shouldn't being overpowered by women when you have the build to overpower them yourself lean more heavily into the femdom kink? Well, I find femdom titles with that style of PC more enjoyable at least, horrifyingly rare as they are.
Anyhow... I figure the reasons for boyish protags don't go beyond trending, aka game devs copying popular titles that others have previously looked to for successful titles. Very rarely do developers take steps outside of what's current, and of course that applies to H-games too. There's additional factors to consider, like artists having a general bias towards drawing characters in a particular style, age range or build. Very few of them don't get locked into this mindset but that's what happens when you draw porn for a living.
Putting all of that aside, I'm just glad the brave few got male protag H-titles relatively popular in the first place. I love seeing an actual cast of attractive enemies rather than hastily scrawled unsightly monsters the artist clearly seems to have resentment towards drawing.