Let me deal with it. I’m a grown-up. After all, guys flirt with me when my sweetie’s not around, and I manage to deflect them just fine.
The idea of my husband stepping in to “rescue” me is kind of ridiculous. When guys hit on me, I either tell them to buzz off, or I enjoy the attention for a little bit, and then tell them to buzz off. I can’t imagine why my husband would step in.
Really, I’m not some sort of defenseless flower.
Like others say–what do these chicks do when their boyfriends aren’t around?
An insistent or obnoxious guy would be treated like any other rude person. I’d tell him to go away more forcefully, and if that didn’t work, I’d say something to whomever I’m with.
Well, it all depends on the people involved and how they view “flirting.”
If some guys walks up to me and starts chatting in a normal way, that’s not flirting. I don’t need to be rescued, I’m chatting. If you attempt to help me, I will be unhappy.
If, however, it’s “Can I buy you a drink?” or “Can I get your number” or “Can we go out sometime,” I will explain that I am not interested. Again, I do not need “help.”
If the guy is threatening, drunk, or scary, then feel free to help. But make sure you only get angry at the guy and only if he deserves it. Unless I’m flashing my boobs, being friendly isn’t “asking for it.”
But everyone’s different. If you’re thinking of a specific girl in particular, ask her.
Fuji Kitakyusho:
Hee hee hee hee…
Oh that’s evil 
I prefer to handle things myself but my husband always keeps an eye on me for help signals.
There’s only been one guy that caused him to bolt to my side in the nine years we’ve been together – and that was because the guy in question was an acquantance and he pretty much told hubby that he was going to hit on me, walked over and gave me his business card in plain view of hubby.
It would have been hilarious to watch had I know what was going on – the two of them, unwilling to say anything about it in front of me, hubby staking out his territory and the guy testing boundaries all while being fake jovial to each other – but I just sat there with my mouth hanging open wondering why they were acting so weird, my head going back and forth like I was watching tennis.
I think CrazyCatLady has the best suggestion. Find out what your girlfriend prefers. It’s really a personal thing.
For me, I prefer to handle it on my own at first, but have my SO step in if the guy isn’t getting the hint. Usually, I make it pretty obvious that I’d like help by waving him over or going over to talk to him (my SO).
You probably can tell from your SO’s body language whether she’s enjoying the attention or not.
If you can’t, time to get a crash course 
would the answers be the same if the question was “Do I step in and make it clear he’s not available?”
Like a few others have more or less said:
It depends on the situation. If the guy seems fairly harmless, I’ll try to handle it. If the goober just isn’t getting the hint or things look like they could turn ugly, it’d be nice to have a bit of help.
I don’t think I’d want my guy acting like some jealous lug everytime another man flirted a bit. It’d get tiresome and damnit, it’d be nice to fend for myself sometimes, y’know?
I think I’d treat him the same way… I’ll let him handle things if a woman flirts, but if she won’t get it through her thick skull that he isn’t interested, I’m not going to sit by and let her pester us.
I’m lucky in that Airman isn’t the jealous type, and we seldom go anywhere where there might be the potential for men to hit on me. I have noticed, though, that there have been a couple of occasions where he put his hand on the small of my back as kind of a possessive gesture.
However, I have run into a few clueless individuals who ignore the large gold rings on the ring finger of my left hand and try to ask me out anyway. My usual retort is, “I’d love to. What time should my husband and I be there?”
Works every time.
Robin
Bingo ! CCL got it in one, as usual. Different women will want different things, so why don’t you get to know your girlfriend a bit better so you can tailor your response to that specific person ?
As for me, unless the guy is getting physical or I ask him for help, my husband better cool it. And the same goes if the situations were reversed. I’d only step up if he made it crystal clear he wanted me to. I would be extremely angry if he tried to ‘help’ or ‘protect’ me, unless requested. It’s got absolutely nothing to do with ‘feminism’, either.
I know women who have the completely opposite point of view though, so as I mentioned earlier, it would be better to get to know your girlfriend and how she’d react, than a bunch of very diverse strangers.
I’m one of those socially awkward people who just cannot deal with being hit on, generally. I always get awfully uncomfortable and I have a very funny story about how I once tried to subtly let a guy know I was taken.
I sincerely doubt my hope if he did he’d do exactly what cowgirl suggested: casually coming over, saying hi, a quick kiss.
But I never get hit on, so it’s a moot point.