By calling out sexist banter and rape jokes every single time they hear it.
Fine in theory, but what following scenario is more likely:
Scenario 1
Pissed bloke outside pub: "Oi love, show us your tits"
Other random male pub patron: "Come on mate, no need to say that, it's really disrespectful"
Pissed bloke outside pub: "Hmmmm, yeah, sorry you are right, that was bang out of order. I will be more considerate and respectful in future, thanks for the heads up "
Scenario 2
Pissed bloke outside pub: "Oi love, show us your tits"
Other random male pub patron: "Come on mate, no need to say that, it's really disrespectful"
Pissed bloke outside pub: "Fuck off you prick"
Other random male pub patron: "Seriously, it's not cool to treat women like that"
Pissed bloke outside pub: "What the fuck are you going to do about it..................oi I'm talking to you.....come on then.....<sound of glass breaking>"
etc etc
It's not a case of not wanting to challenge crap behaviour, it is a genuine fear of what the dickhead you have just challenged is going to do.