I know the title might sound ridiculous, but bare with me.
Me and my girlfriend are both 18, and have been together about a year and half now. We've had our problems but are normally quite good at communicating and working through our issues.
However, for about 4 or 5 months now, there's been an issue we can't solve. My girlfriend lives with her roommate (18F), in a flat just the two of them. About a year ago, this roommate (whilst still 17 years old at the time) started engaging in a sexual relationship with a 30 year old man from her work. We discussed how disgusting we both felt this was, but he lived far away at the time and they rarely saw each other.
Since about 5 months ago towards the start of the year, he moved back much closer, since then her roommate and this man have taken their relationship to the next level and are now officially in a romantic relationship.
As such, he now spends 3-4 nights a week on average (occasionally even more) over at my girlfriends apartment. Now, as far as I see it (and my girlfriend agrees) he is a disgusting man, I'm not sure if it would actually be illegal in our country or not, but I will never condone a relationship between a fully grown adult of 30 and a child of 17. I am completely and utterly uncomfortable with him being there, being anywhere near her, talking to her and even the thought of him looking at her doesn't sit right with me. It stresses me out to a ridiculous extent.
This is where the conflict comes in, on nights when I know he's over and I'm not, I get ridiculously stressed out, which can lead to me taking small worries out on her in ways I would never normally (which I completely understand is unfair and something in trying to work on) or asking numerous questions about his wearabouts in the flat, has he seen her or spoke to her or done anything weird towards her etc. She says that this makes her feel trapped in her own home, and like she can't even leave the room without having to comfort and assure me, which is getting too much. I completely understand that and I do not wish to make her feel that way under any circumstances, but I simply can't regulate my emotions with a man like that being in the same living space as my girlfriend, and quite frankly it worries me sick a lot of the time.
Because of my feelings towards the situation, and the way she feels when I ask the questions I do on the regular, it's beginning to become a real issue for us and overwhelm us both.
I've spoken to a few of my friends and people at work about the situation and they all seem to agree with me, saying they would absolutely not be comfortable in the slightest, and I've had multiple people even saying they would attempt to solve the situation with violence directed at the man. But we're not sure how to cope with this at the moment. I know it's a weird situation, but I want to know any advice people on here would have.
TLDR: My girlfriends roommate has a very questionable partner who is frequently over, and him being so causes me immense amounts of stress and as such conflict between us, even though neither of us are in the wrong. We're wondering what could be done to help this situation.