Let's backtrack a bit and focus on the ecchi things.

They say he would **** them all, they say they would had

Well, they were right!

"Noooo! We can't get married anymore," Issei heard the same phrase a hundred times ever since he use the Ownership, this time though instead if the Kendo club, he decided to play with the swimming club members, by overtaking their pool, adding the rule of him being the coach and they have to listen everything, of course, he had made it so that every female is naked, and line up with their ass in front of him, He uses it as a practice on his finger writing.

Dozens of writing on their bodies, as he keep practicing, he had reach to a level where he could write as fast as he swings his his arms. Efficiency maximum. He eveb had butt sex with those that have the best ass in there respective cclub.

The men in the club called him the god among men and worship him when he told the girls to swim naked as they can do nothing but masturbated as he is wrap around by wet naked girls of the swimming club.

Though by Sona's advice, Issei does made them forget it all that he ever did it(sometimes its easy to forgot how powerful his ability is). As this is nothing more than the writer getting bored and decide to make scene without any plot relevance.