A lot of my guy friends are my boyfriend's friends, too. The reason for that being is because I've only lived in this town for two years, he's been here his whole life. So when it comes to watching movies with a guy friend, that guy friend is more than likely his friend, so there really isn't ever an issue.
I guess him & I have an understanding of each other, because we go out to the bars without each other, no big deal. I trust him, he trusts me. If we ever have an issue, we address it & take care of it.
However, if this weren't the case, I definitely agree with Poetic Justice. If I really care for my SO, & if I don't want to upset them over something petty, then I just wouldn't hang out with this other guy alone.