User blog comment:FrankieX98/The future members of the Issei's harem and peerage./@comment-31137559-20180908064945/@comment-2601:87:8300:20D0:5584:F23B:D5B5:DAFA-20180908140202

Yeah right Issei can't do it because he was never the type to force himself on a  girl  and Grayfia is too devoted to her husband that she sees Issei as a little brother. So he needs an older women to teach him. If not Kuroko, I bet Yasaka would tease him, but I can see Roygun doing it otherwise it won't happen.