Japan's not really open to LGBT so maybe they think that it's better for Jun to know when he's older. If my memory serves me right, the couple has yet to come out outside S.O. so maybe, they want to keep it that way until they're much comfortable? Judging, Jun could accept them both as his parents but he'd be too young to understand that not all people think the same and he'd blabber it out. Ultimately shunning him from making making friends, bc some people arent open with same sex couples.
i understand what you say but they don't have to treat like he is his mother, i know he wants to be his parent but i think if they wouldn't said this jun would still consider him as a parent, they can pretend treat like friends sharing the same house, anyway, they lie when they're in that costume and they don't look like an overly couple when they're in the costume.
would* he is*
why they hide his gender from child, he is not gonna devastated if he finds out he has two fathers