the reason he couldn't get alone with any of those women was bc he liked someone else all along, and just because he likes one guy doesn't make him gay he might just be bi and never explored it.
I think its pretty obvious that its not only ichika who feels something for shuu. Right from the start, the way shuu never got to last long with his girlfriends, the way his co worker told him he talked about ichika more than his own partners and lastly, when he was thinking about his "ideal future" where he and ichika have their own families, he said he couldn't picture it alone. Shuu definitely harbors feeling for ichika and he just didn't realize it yet because he was so blinded with his own definition of "ideal future" and "normal"
i'm not convinced that one can go from exclusively sleeping with women to sleeping with a man at such speed of light. it's not being homophobic, but one needs time getting used to the change of pace and that didn't happen here. i know i know, who looks for logic in a yaoi manga... still