Perception?

I'm someone who is fortunate to have a large social group of friends. I have my fair share of female friends who have been there to support and encourage me for as long as I can remember.

I've noticed recently that many of my female friends look at and treat me differently when they learn I have a girlfriend. I thought it was my imagination but many of them seem curious, more friendly and in some cases, a bit flirtatious. When I was single for a time, this rarely happened at all.

Is it normal that the perception (that my female friends have of me) changes that quickly now that I'm with someone?

I would say so.

I've been in the same boat before, and over the past few weeks I've tried to get into the mind of a female by thinking like one, and see it from their point of view. You know, because my mind wanders and I think about a lot of things. The best I can come up with is that deep down, those girls want to claim you for their own. It's not necessarily because they like you, but now that you aren't available and limited to only one person, you're limited edition. Guys in relationships must have some redeeming qualities, or else they wouldn't have gotten into a relationship in the first place. So naturally, they want to try and get into your good books (not necessarily to try to NTR you) in the event that you two break up. If that rings true, then that would explain why paranoia regarding cheating is common in relationships.

If you two break up (which I hope you don't, just an example), you might notice you'll start getting more attention from them and they will act like they care. After a while though, they'll just revert back to what they were like before you got a girlfriend, when they realise that the guy they're after is still grieving his freshly-ended relationship and thus, is not ready to get into another one.

I don't know why it happens, but if I had to take a wild guess, maybe these girls see "taken" guys as a symbol of safety, warmth, love, kindness and just someone who has the potential to adore them, especially if the guy is good natured and wouldn't say "Boo!" to a goose, which is something all girls secretely crave.

I'm not the best with words, but I tried to explain it how I see it as best I could.

probably the best explanation for this

sounds just about right sir.

They see something can't have.
Even if they didnt want you before they do now, just to be greedy.
At least thats how I've seen and experienced it.

Yes, this is completely normal.
And I'd say... between 50-75% of the time it's completely platonic.

I think it's a basic territorial response, pretty standard. Mostly it just means they care about you and if they consider you family, then they want to make sure your new partner is suitable mate-material. My only advice is to be respectful in honoring your bonds with your friends, as well as your bond with your partner. Make clearly defined boundaries. Other than that... there's really nothing you need to prepare yourself for. :3

Sometimes they just see you in a new light. Sometimes forbidden fruit.

Just hope it's the first, unless you like the second one.

Its perfectly natural for that to happen. Its because there is now limits on where your relationship with that person can go. Theres now boundaries. I treat my guy friends waaaaay differently ( I cut down on the sex jokes) out of respect of his new lady friend

As a single guy, they didn't flirt with you because they didn't want to lead you on, and maybe ruin the friendship. But when you became involved in a relationship, it became safer for them to interact with you. If you have a break up, be prepared for things to return to the way they were - unless one of them starts seeing you as a possible catch. Just don't let them distract you from the woman you should be paying attention to the most! I believe the new attention you are getting is more of a subtle signal, from your friends to your new girlfriend, telling her how lucky she is to have you. I would take it as silent support of your relationship, and nothing more.