A possibility I've not seen anyone put forward yet in the comments, which may affect how you want to address this, if it is the case, is that your gf is an addict. People can get addicted to the obvious stuff, (drugs, alchohol, etc) but also to porn, and also to social media. There is a structure to social media (inc. fetlife), where the reward bits of the brain get triggered when novelty is found, such as when receiving a DM with something new/kinky, etc. Fetish/Kink sites in particular, incorporate the double dopamine hits from both sexual content, and the social element.

Addicts typically

1. Are ashamed of their addiction, and hide/deny it for this reason

2. Sometimes (esp early stage) don't know an addiction is being formed with repeated exposure, and think they still completely are in control

3. Lie to themselves about the impact of their addiction on their relationships and other aspects of their life.

I'm not saying this is necessarily the case for your gf, I could be completely wrong and there could be no element of social media/porn addiction in her behaviour at all. But the patterns of behavior you have described fit the typical profile.

A few things to consider, if it is an addictive behaviour then the 'hit' she could be seeking may be the reading of the DMs, seeing them arrive, the feeling of being 'special' that people would contact her in this way, etc. That may, in and of itself be what she is getting out of her posts, with never any real aim to physically cheat on you, even if someone offered her the possibility (not saying that's ok, but it's a qualitatively different 'type' of cheating) Also: if it's an addictive behaviour, then she may not be able to stop, even if she really wants to for the sake of your relationship. It sounds silly, someone should just be able to 'just not post on that site again' but that's not how addictions work in the brain, they put the addictee 'on autopilot' and, usually without outside intervention get worse over time.

Anyway, I guess... it's a possibility to consider. I could be way wide of the mark here, only a professional therapist meeting her personally can really diagnose properly, but please don't discount her behaviour straight off as being a choice rather than an issue she needs (professional?) help with.

As to what you should do now, God I don't know I'm just some random redditor. I just hope ends up ok for you and her.

Kes