Well, for one, I play role playing games. So, I like to experience different roles, different ways of thinking, and playing as someone like FemShep or female V in Cyberpunk offers a much different perspective on life than if I played as a male. I'm a male, I go outside as a male, why do the same thing in gaming?
And for a second reason, if I'm playing a third person shooter why would I (a straight male) want to stare at a man's ass for 120ish hours?