I talked to some of my friends from the Boards about this topic and what I've written below is a compilation of ideas/thoughts/points that we all agree on.
Yeah, so, this becomes an interesting situation and it's one that's incredibly to give practical advice about because a) it's nuanced and case-by-case and b) the guys who find themselves in that situation can't generally apply the correct advice I (or any other advanced seducer) give because, if they could then they wouldn't be in that situation in the first place.
So, that aside, there are a few concepts to understand.
1) If you're in a situation where your girl is already fucking another guy behind your back then you fucked up somewhere early on - at that point, you're not giving her permission or influencing her decision because she's already doing it. At this point, you either leave or you go with something like "Oh, I guess we're in an open relationship now" so that you can also fuck other women but, I'd probably leave.
2) If she's just texting a guy and let's you know then it's probably a test to see how you react - it's a mechanism to get attention from you, if you pass the tests then she'll realize she can't get your attention by talking about other guys. If you respond with anger/jealousy/investment then she learns "I can get what I want from him with threats of another man being around."
3) In all situations, I don't care what my girlfriend is doing. If she wants to fuck another guy or text, etc. that's totally up to her; she just has to be fine when she's no longer getting my attention (in the same capacity), and she has to be fine with me not being committed to her anymore. If she wants to just fuck keep fucking me while I look for other prospects, cool.
That's why this is tough to talk about with guys who don't have a ton of experience because the points above wont' fully click for you unless you have massive amount of experience under your belt. The point is, I don't own my girlfriend and I'm not going to take any measures to prevent her from doing anything - I know my worth/value to a woman and I can approach/fuck her brains out so I'm providing some of the most important things a woman can find so, by that nature, women WANT to keep me around and will not take risks to lose me. The only prevention I have against my girl fucking somebody else is being her best option and, generally, I am.
Hope that makes sense for everyone.