User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-178.128.252.204-20150826194658/@comment-77.105.22.24-20150830095235

Rias is legal wife so she won't have problem with other girls having sex with him. She knows that she is speciall to him and that he loves her.

Issei is no longer human btw. He is true dragon-devil hybrid, heck nobody is even sure. Cao Cao asked him that in volume 12 too. He is not equal to Vali simply because there is nothing that can rival Diabolos Dragon. You don't stand a chance agains infinity (Rizevim confirmed this :D) that even  Vali will get his ass kicked.