Men who have GFs, what’s your opinion on your girl having a straight guy best friend

As the title says, I’m curious how others feel around a situation like this. My Ex would talk to her best friend everyday and I’m pretty sure he’s in love with her. And she openly told me she wouldn’t give him a chance if he tried anything, which he tried to ask her out once, and they are still best friends.

I always told myself not to worry about it, but it always bothered me in the back of my mind.

Now post relationship, in curious if others have had similar situations or feelings? Let’s discuss. Thanks

Edit: for more detail

For more detail, I have met the dude multiple times and something had always seemed off about him and her together, just the small conversational bits I’d pick up on. And how she would communicate with him opposed to how she would talk to me. She’d be more critical with me, but more understanding with him, and would actually listen to him, rather than not listening to what I have to say about situations.

It’s hard to put the situation into words, so I apologize for the lack of replying I’m doing.

Thank you everyone for your insight, experiences, and replies.