I can’t comment on every prison just mine however it doesn’t happen very often, that gets reported anyway.
I’ve caught two guys giving each other head in the showers but it was consensual. Also caught two guys in a double pad having sex but again consensual. There has been allegations of sexual assault but it’s usually by the LGBTQ+ prisoners and when investigated they turn out to be untrue/over exaggerated as to what has actually happened.
I’m not saying it doesn’t happen though because it does.. I’ve just not came across that in my career..