Simply answered, no it's not f*cked up.
As long as it's 'some dude' and not your friend's girl.. or maybe that is okay depending on what you define as a 'friend' and what you expect in return from them.
You see it happening all the time; girls are with dudes who just can't satisfy them. They may have liked them for their charming abilities and social status maybe at first, but now they are starting to find out that there are others out there who can offer them more. It's totally natural and all it is is the survival of the fittest. Hell, sometimes it's just that that particular girl is a sl*t and wants some strange.
My friends and I like to put it like this way; if your girl is attempting to branch swing infront or behind your back, she's no longer yours anymore; she's whoevers for the taking.
It's brutal but it's an honest statement. This is especially true with young women who believe they can find a 'stronger man' or a 'better f*ck' than what they already have out there.
Ultimately they will get what they want and they will f*ck whoever they aspire to.
Sometimes they enjoy it as much as they thought they would and sometimes they are disappointed. When they are disappointed, they crawl back into their little shells and attempt to go back to what they had before (their boyfriend or ex boyfriend depending on if she's been caught) until the next possible branch becomes visible.
It's standard behavior and a lot of AFCs can't understand it.. it's exactly what it is; competition; survival of the fittest.
Some women are bigger branch swingers than others; I haven't really found what separates the bigger ones from the smaller ones, because I've met confident women who are both.
Bottom line is it's not his anymore; it's yours. Just take note that she is a branch swinger and she is swinging to your branch. Realize that she may stay there for awhile, but there's always another branch, higher than yourself.