TRP is spreading.

If you think this isn't an unpopular opinion, try posting this in any relationship subreddit and prepare to be relentlessly shamed for being "an insecure incel loser blah blah blah."

Whenever there is a friendship between a man and a woman, at least one of them (usually the guy), wants to have sex with the other. Most of the time they won't openly admit it, but it's the truth. Now perhaps a guy can be friends with a girl and refrain from making advances, but typically the guy would jump at the first opportunity if his female friend made a move or was emotionally distraught from a breakup. My point is that at least on person almost always wants to have sex with the other. If a guy is routinely hanging out with a female "friend" who is in a relationship, he's only doing it to have his foot in the door so he'll be first in line when the relationship ends. Most men know this shit, and women tend to be more naive about it.

It does not make you insecure to be untrusting of your girl's "guy friends", because typically that guy wants to get with your girl but is just waiting for the right moment to make a move.

Can men and women be platonic friends with zero feelings? Yeah, I think so. However, those instances are so few and far between that it would be incredibly foolish to give other men the benefit of the doubt and expect them to respect your relationship. Most people don't give a shit about you or your relationship. I'm not saying that you should prohibit your partner from hanging out with the opposite sex, you're not her keeper and she can do whatever she wants. However, it's important to know that if a guy is hanging out with your girlfriend, especially if it's one on one alone time, he is almost certainly not in it for platonic reasons.

Edit: I should have mentioned that this really just applies to alone time with "guy friends", hanging out with guys in group settings is different and I don't see any problem with that.