Well, I'd have said it was pretty close to the legal thing to do, too.
The "self-defence" justification doesn't just apply to defending yourself but also to defending others and, somewhat to some people's surprise, to defending your property too.
So .... :-
- he tried to intimidate you with threats. Given his proximity, that's almost certainly assault.
- he tried to bop you one and you bopped him back. Self-defence, and minimal force.
- he tried
again, so not having got the message, you bopped him again and kicked him in the plums for good measure. Then left. Again, pretty much minimal, and certainly (
IMHO) reasonably, force.
Up until that point at least, your actions are pretty much textbook legal. Perhaps you could have then called the police, and at least they'd have known to be looking for a wannabe rapist, but given that the woman had cleared off too, there wasn't much they could do about that and, you're right, you'd probably have had to extensively explain the whole thing. The pragmatic thing at that point was exactly what you did.
So I'd say it was the right thing, and pretty much the legal thing. Was it it the safest thing to do, though? Perhaps not. He could have been stone cold sober, armed with a knife and quite willing to use it, on you or on her.
And
that sums up my response to the original question - it all depends on the exact circumstances. What's right and legal may or may not be the same thing, and may or may not be clearcut, all depending on the situation. And sometimes, by interfering, you may make things worse. But personally, I'd
always call the police first, unless the minute or two of delay could be catastrophic, but frankly, if it's that bad, I'm probably not going to want to jump in the middle of it anyway, so maybe still hang back and call the professionals.