I don't have an issue with swearing in general, it's the aggression that often comes with it. So, if someone stubs their toe and utters a string of "cunting fucks" as they weep in pain then I can live with that and explain it to my child if necessary (Don't think a child will notice unless it's pointed out to them).
Some teenagers (and adults) swear on buses etc in a "I'm so hard, I can do what I want, no really, I am" kind of way which can leave you wondering where the situation is going to go. I'd assume in most cases that they're insecure, but insecurity doesn't mean they won't turn nasty if challenged and that is what I personally react to. The implied threat.
It was a drunk teenager who called me a fucking cunt and shoved me over because I wouldn't let him insult me and my friends without being told to back off.
I've still challenged people who are being aggressive because I'm buggered if I see why I should creep around being scared and letting them have everything their own way.
I always enjoy Stephen Fry's activities and his thoughts on whatever he cares to give his attention to, but I've read what he had to say on swearing and I don't think he's talking about that part of the swearing spectrum, the insecure, aggressive swearer who just wants to intimidate and big themselves up.
I'm no saint, I swear mightily when the occasion calls for it, but not as a rule in public, or in front of my child. I don't want to scare or upset anyone.
So to summarise 
a short burst of public swearing because of stubbed toe/sudden bad news or an thoughtful and isolated "oh fuck" doesn't bother me, aggressive swearing does and IMO the OP was perfectly within their rights to challenge it on the basis that groups of swearing teenagers can come across and frightening and aggressive.