Funnily enough, for me all I had to do was stare/ play with her ass and MC stood at attention again. Boobs also worked.Read my above post for how you could deal with that bug.
For those struggling to penetrate, make sure you aren't max size with maxed out unit size too, 'tis too big man. Then there's the resistance/ pain tolerance that you "could" adjust with monkey, now you gotta get her "wet" first. The ass chicks are easiest to please and open up, since they get excited via exhibitionism... or I got lucky with my spawn.
So yeah, MC got so much ass in previous updates, he now requires our assistance to "align" his interests. The downside of a one-track mind. (Stare or rub bare body and it gets stiff, if you struggle to comprehend)