User blog comment:B214/Kuoh Academy students finding out that Issei stays with the ORC girls and is dating Rias./@comment-24098614-20131215011722/@comment-16281343-20131215085536

Well, I did accept the fact that the girls might know that Gasper's a boy b'cos of that volume but not the boys.

Like I said, yeah M&M might also know, but even if they do know, not the other boys.

If the three of them know, ofc they won't include Gasper in their conversation when talking about ecchi things.

M&M can't prove to others or specifically the boys that Gasper is indeed a boy.

And if they make a campaign to prove it, Issei will just prevent them from doing that to protect Gasper from harassment, since Gasper will just freak out from all the attention that will give to him.