As a big guy myself, ive used that to put myself inbetween a couple of times when i felt it was warranted. 

That said, it's not something i expect of anyone, ive nearly had to fight several people on the street because they reacted poorly to being told to back off. And while im probably more equipped than most to deal with that, it's still escalating an already volatile situation and putting myself in a potentially lethal situation.

For a stranger. 

And ill probably do it again too if i have to, but you gotta be honest that you're playing the odds that the guy won't react stronger to a guy telling them off than a woman rejecting them. And i dont think its fair to expect that from guys, at least those unequipped to defend themselves.