Is it okay?

Is it okay to stop seeing a guy because I don't trust his relationship with my best friend? 
Here's the situation- my best friend has a boyfriend. His best friend is the guy I'm seeing. But my best friend is very good friends with him. And he's said he thinks she is beautiful. They have dinner together and cook for each other, and watch movies together and I trust him. I just have this gut feeling my best friend is going to cheat on her boyfriend with the guy I'm seeing. 
I've tried to bring the topic up to sus things out and she seems jealous of how things are going with the guy and I. 
I've had heartbreak and I'd rather not go through it if I can help it. So do you think I am justified for not seeing him anymore- if I decide to end things?

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