I feel like sexual compatibility is rarely brought up like this in NTR. She really does love her boyfriend but the man is basically a passionless eunuch.
Reminds me of how in School Days the MC's first girlfriend is too shy to do more than hold hands after weeks of dating.
I wish there would be a boyfriend redemption arc here but his inexperience and small penis all but guarantee the girlfriend is never going to be sexually satisfied from the relationship.