@Genesis7478 I think it was already kinda explained.
Jun is obviously not actually Rio´s child, and we can say that either Jun´s dad and Rio got together wen Jun was still very small or that something else is at the work, whatever happened to Jun´s mother is still unknown, thats for sure, it could be that his Mother died at birth, she left them, or we might be looking at a "surrogate mother" type of business, or even a "mistake" pregnancy, hell, Jun actually being the son of his Father sister isnt far-fetched too, adopting nephews isnt that rare in manga , whatever the case, this obviously puts pressure on Rio in every type of way, he doesn't want Jun to stop accepting him as his "mother" if he learned that the person under the suit living whit him and his father is a man, and he clearly loves both Jun and Jun´s Father way too much to take the risk.
Obviously, for us readers is clear that this is just a silly side-drama, as from just 6 chapter we know that Jun would probably not really care, but yea, from what I got, this is the current reason behind the suit, maybe there was a actual situation that prompt him to start using the suit (like chit-chat behind their back or something like that), but yea, this will most likely be better explored later on.