User blog comment:B214/Volume 17 spoilers./@comment-117.193.172.96-20140220163805/@comment-1359322-20140221002007

M&M were never banned from entering Issei's house more like they can't even banned them, it'll be seen as rude by outsiders for kicking out your guest.

As to why Issei doesn't want to reveal it, it's because he wants to be able to date Rias in peace, if it gets revealed, the students would undoubtedly stalk on Issei and Rias 24/7 under the context of "protecting Rias from the Wild Beast Hyoudou".

And also Issei wants to maintain a peaceful school life too. Issei is already fighting many powerful opponent, even if the school student are weak (actually there might be powerful ones too, considering that some of them aren't human or are humans related to Supernatural beings), being attack everyday in school isn't some fun or something Issei could endure in the long term.