Hi, so I was at a party the other day. Met this cute Italian au pair, and we hit it off. I escalated a bit but not too hard. I was touching her back, and her arms. We talked a bit about growing up, joked around about a bunch of things. I took her to get another drink with me. At this point this other dude showed up and tried taking to her, but I made it clear I was with her and eventually he left.

However, at some point I had to go pee. So I went to the bathroom, got her number and she said she was going to find her friend.

After I got out of the bathroom, I see this guy chatting her up. They basically ended up taking for an hour, he stayed at the same place I first saw him, but he was aggressively kinoing her.

It really pissed me of that this guy got in. She ended up leaving the bar with him, but idk if anything happened. She ended up texting me and asked if I wanted to grab drinks. Now I know it sounds like something happened with this guy, but the girl said that she’s only had one one night stand in the past.

How do I prevent guys from doing this like this again? How would I make it clear that a girl is mine and that another guy shouldn’t try to fuck her?