How is it that men can watch a video of a sexy woman doing naughty things and they can be ready to go but when the roles are reversed and a penis is put in their face they freak out?

This is something I don’t understand at all. I can understand if they’re gay but even when that isn’t the case it’s still a freak out. So these women are on the internet and in videos but why is it so different when the situation is reversed. I’m just asking for my own understanding