Hey Reddit, long time commenter, basically first time posting. Me and my girlfriend have a unique problem that's happened 2 times so far in our relationship. We enjoy our alone time together, but sometimes old friends will pop up or we'll make a new friend, just like any other couple. However, recently we've basically sworn off having male friends because of the issues they've created by trying to cause a rift between us. I'll go over each incident.

• C: This guy went to high school with my girlfriend and had a known history of getting obsessive with girls, but usually had respect of other's boundaries. We gave him a place to sleep for a while as he had gotten kicked out of his place, and he seemed like a pretty okay guy, if a little brag-y and egocentric. Somewhere down the line, after about a month of him staying with us, I had left for a couple days to help my grandfather with some things. Now, during this time I was in constant contact with my girlfriend, not by obligation, but simply because we enjoy talking to eachother. Suddenly, C texts me saying that L (my girlfriend) has been cheating on me the whole time. I guess he'd taken opportunity of my absence to lie, but L was visiting her grandma's while I was gone, and C stayed at L's mother's.

Now, this guy seemed a little strange around her, so I took this with about a pint of salt. I asked him for proof, and he didn't provide anything substantial. L was sleeping at her grandma's (I can't remember the reason), and I took a cursory glance at C's Facebook for a reason I don't quite remember (this was about 1 year ago), and I found a picture of C and L in typical selfie fashion. Confused, I decided to drive over to L and question her. Now, I'll admit, this was the first time I'd ever experienced something like this, so my anxious thoughts sort of caused me to almost begin to believe these things. I was a little angry when I arrived and, while I didn't do anything destructive, I was a little rude.

Turns out that the picture was from a bonfire we'd had a while ago, and was intended as an innocent picture with a friend. So we cut him off and went about our life.

• Co: This one really fucked up my trust with other people. I'd been best friends with this guy in high school, one of the funniest, nicest dudes I'd ever met. We stopped talking for a while as high school graduates usually do, but eventually we met up and started hanging out again, and things were great. We'd hang out at our apartment and the three of us had become good friends. Unfortunately, due to an emergency and bad budgeting we lost our apartment, and begun staying over at Co's house until we got back on our feet. This is where things got weird.

Things were pretty great at first. We hung out all the time, had fun, etc. After a time Co would start...snuggling L, I guess? I didn't much like that, and I should have been direct, but I'd play it off as a joke like "hey dude, back off my girl". I know I should have just told him I didn't feel comfortable with this, and at the time I also knew and my self esteem dropped because of my passiveness. Eventually, I get tired of it and move out to my parents. She had to stay with him because we have a cat and the only place that would allow her cat was Co's house. It was a small sacrifice to make, and I voiced my concerns to L, and she told me not to worry. I do trust her, she's never given me a reason not to, and she had no reason to do anything shady.

Then, as before, Co now claims they had been having sex. This blew my mind. I didn't think I had any reason to distrust him. He had been my best friend for years and years now. After I said I can't be associated with him anymore we cut him off, but he would text and call L daily. She moved in with me at my parents after we learned that we actually could keep the cat there as long as it stayed in our room. He texted her things like "I love you" and "I miss you", as well as trying to call her constantly. This went on for weeks until she grew fed up and changed her number.

So, I don't understand this and I'd like some input. In the almost 4 years we've been together, this has already happened twice. Is this just bad luck at work?

TL;DR: Seems as if anyone we make friends with has no respect for boundaries or our relationship, and I'm looking for outside input.