Some context:

I'm 20, studying at Uni, and 'should' be moving in with my girlfriends in 2 months. About a year ago I began working on myself, and I've been with this girl for 6 months; she's my first girlfriend and so far it's been a good learning experience.

 

She's been getting pretty close to this guy recently. Spending time revising with him and some others, going bowling just with him, and the other night she spoke about how she'd be perfectly happy with an open relationship. She was also on Tinder for a few days without my knowledge, but claimed that it was a 'game/competition' she was having with some girls at college.

Also, she brought up how she'd be happy to have a MMF threesome, but she'd have to get to know the guy first (because that's some justification, pah!).

Now, in the first few months of the relationship, she was pretty obsessed with me. I told her from near the start we would never be getting married, that I'd always be skeptical about cheating (through my understanding of AWALT and proof of this through my parents' downfall of their marriage), and that communication is important - I won't stand for shit so if you've got something to say, then say it.

The awkward bit is that I'm supposed to be moving in with her. The place is a short walk from my placement (I'm taking out a year in industry to work for a company) and, since my place of work is in a town, there isn't any student accommodation and I'd have to be in a house share with a random, older person paying a good amount for rent if I was to not live with my girlfriend.

 

So it sounds like I'm sacrificing my dignity and self-respect for the convenience of having a cheaper, more convenient place to live for a year? Well, that's somewhat accurate. However, I enjoy being with this girl and it's not like this relationship isn't fun, because for the lat 6 months it's been great.

So, what are my next steps? Confront her about these things, tell her I won't stand for it, try to be as RP as possible and threaten the relationship? Or just hard next?

What frustrates me is how, when I've brought up how the 'jealousy'/skepticism I feel when she's gone bowling with this guy, or when she's brought up talking about an open relationship with him, then she says stuff like "I've never neglected the idea", or "You're just overthinking things". Yes, this sounds like bullshit, but is it not natural for a girl to get bored in a relationship when it 'stagnates' and start thinking about these sorts of things? What is the correct course of action when it does? I don't want to throw away a girl because the relationship stagnated and she's began feeling attracted to other guys, however I won't allow myself to continue to be a little bitch and accept things I'm not happy with.

 

Update: Thanks for the responses. She came over after many voicemails, messaging friends etc. I told her we can talk for a bit then she leaves. We spoke, I voiced what I had to, and she cried for about an hour. She admitted to the signs of disrespect, apologized, and explained the red flags. I emphasised that this is the first and only chance.

I feel like I've gone against what I believe in by staying with her, and I respect that all of you guys would have hopped out of this relationship in an instance. I guess that, since it's my first relationship, I've got things to learn still, and crushing things (like red flags) as soon as they appear is something I need to do.

I still struggle to differentiate between genuine signs of emotion and care, and manipulation. Receiving texts that tell me that she'd do anything for me and having her cry to me for an hour tells me she cares, but then there's that niggling feeling of 'this will bite you in the ass'..