My gf and I are still fairly new to this. She keeps going back and forth on whether she would ever want to have sex with someone else or not.

She has now started talking to this guy (who is not a direct coworker but he works in the same company as her) and they get along well. Yesterday they texted for hours and she likes his personality and also gets very nervous around him... (what actually makes me feel weird). Today she texted me how she can't stop thinking about him and cannot concentrate and that she thinks about meeting him up - also potentially in a sexual way (they have only talked shortly in person at work weeks ago and other than that mostly texted yesterday and she suggested grabbing a beer to him). tbh I don't find him that good looking and I'm not sure whether I would get excited by the thought of him and her because of that but I haven't told her because she was against the kink in the first place and now that she finds someone exciting I didn't want to limit her right away. I believe once I would know him also things would change.

I texted her today that we should take things slowly and talk again about how far things could get (emotionally and or physically) so we both feel comfortable and she agreed that we should take things slowly. Days ago she mentioned that she would never date someone like him ( she barely knows him other than the texts from yesterday) so I asked her why she could never think of dating him and she asked back whether I would want her to date him - with the smiling emoji that has the water drop right to its head. I replied that I would not want her to go on a date with him and that I wouldn't want her to accidentally catch feelings ...and that we could think of how to prevent that. (we are mostly texting rn since she is not in town)

anyways...I had read how many couples here do not let their gf/wives go alone to dates with some guy and that it is often times purely sexual. Or are there couples out there who let their wives get to know the guy alone first to create some connection that some women need to feel comfortable for sex?

My gf and I had talked about how if he asks she would tell him that she has a boyfriend (and that we have an open relationship). I kind of worry that this will not go as planned (as he might be a bit conservative)... that he might judge me as the man, as her boyfriend and that she would not be strong enough to defend me or maybe even adopt his views in that situation and become insecure about us (since she is a big people pleaser).

What I would prefer actually is that she and I would get to know him together (as friends first) and see what kind of person he is and if he respects her having a boyfriend, it would probably also excite me more, having seen him in person and maybe even seeing my gf flirting with him in front of me. I would suggest this to my gf when she comes back but I kinda have this feeling that she might find it weird to suggest it to him because he might find it weird, as he could be solely interested in getting to know her (as either a potential partner because he doesn't know her relationship status yet or a casual sex partner)

She by now likes him a bit too much and I wonder if (in a hypothetical situation where I would ask her to stop seeing him or talking to him) she would deep down resent me for having allowed her creating a bond to someone that she she emotionally and physically likes for me to stop it ..maybe she would even accuse me of being toxic or not trusting her.

I thought I had overcome jealousy and I that I now trust her fully (because she had really fought for me and this relationship multiple times ...basically begging me at one point) but somehow I still worry..

What should I do to prevent things from going down the hill?