Am I the jerk for kicking out my girlfriend and her son after she invited her ex into our home?

Me and my girlfriend, Sarah, have been together for four years.

She has a son, Nathan, who is nine, with her ex, Mark.

Sarah has full custody of Nathan with her ex having scheduled visitations one weekend of every month.

Sarah and Nathan moved in with me a little over a year and a half ago,

and from my perspective, it was a little rocky at first. I've always been pretty protective of my

space, so making room for two other people was difficult for me. When Sarah moved in with me,

we agreed that the visitations for Mark would be held in our apartment as long as I'm home.

The main reason I wanted this is because I don't know Mark that well,

and I don't really want him wandering around our apartment without me there.

When we proposed this idea to Mark, he was fine with it.

Now moving on to the real issue.

Three weeks ago, marked four months since Mark's last visit,

so I'd ask Sarah when he was planning on visiting Nathan.

My girlfriend replied by shrugging her shoulders and telling me she'd ask. She never followed up with it.

So a few days ago, I asked when Mark's next visit was and Sarah said she'd tell me when she knew.

Nathan was in the living room but had apparently overheard us and shouted something along the

lines of, Dad was here last weekend, remember? Sarah's face immediately dropped and when I asked what

Nathan meant, she wouldn't give me a direct answer. Eventually, she ended up telling me that for the past

two visits, Mark had been to our apartment when I was working. When I accused her of going

back on our agreement, she kept telling me this was her home too, and she can invite whoever she wanted,

and that it was fine because she was here and watching. I told her that was besides the point, and she

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