It hasn't been confirmed whether Cassidy is a boy or a girl. So it doesn't matter if someone decides they want their Cassidy to be a boy.
As far as lore goes though Cassidy is a girl in the books  but a boy in the game.

It also has NOT been confirmed that Cassidy is the vengeful spirit. If Cassidy is a girl she can't be the vengeful spirit. As the vengeful spirit is referred to as he/him by the animatronics. Now I don't think they're referring to the suit itself as judging the way they're saying it it seems more directed at a spirit.

Michael not being gay/ is straight
While yes in the 80's LGBT wasn't accepted it didn't mean it didn't exist. Plus consider the fact Michael is the only Afton kid who got to grow up which goes past into the 90's there's nothing saying he couldn't be bi, pan, Aro, Asexual. While I don't accept Mike x Ennard I do accept Mike x Noah as long as its not getting sexualized/fetishized.

Something like these

They don't need to be turning into cats, having sex 24/7, kissing every night, dressed in skirts, wearing pink/soft clothing, they don't need to do top/bottom. What if they're both neutral or don't care about dominance in a relationship?