Lets be real, there's usually a 50/50 chance they're at least sexually fluid.
I think sexual fluidity is as common in men as it is women, women are just more open and accepting of it. Men are taught to suppress any homosexual tendencies due to the patriarchy.
They want to scope out closeted gays because it makes them feel like they have some higher knowledge or power ("gaydar") over males who say that they're straight, which ties to internalized feelings of self-hatred that they want to press onto others.
It's natural to wonder if people are gay but the percentage is only about 25%. People like Justin Beiber or Zayn show no signs of being gay and it's most likely just a fantasy or immaturity assuming they are gay. It's a reach thinking every male celebrity is gay.
However, since about 25% of the population are gay/bi, a small percentage are closeted. I believe Shawn Mendes or Will.I.Am could be gay because their behaviors remind me of men I've dated or have been open about their homosexuality.