Basically not wanting your partner to be friends with people of the opposite sex is not a reasonable boundary in a relationship and I do not understand any reasoning beyond insecurities and trust issues for why someone would feel this way.
If it comes from trust, if someone believes that their partner would cheat on them, they should not be with them and no amount of restriction or boundaries would stop them from cheating. If they have not given any reason to suspect cheating I think it's mean to believe and treat them as a cheater or as someone who is not able to stay loyal without having to cut out the opposite gender entirely. If you have trust issues and think your partner may cheat on you when you have no reason to believe that then you should work on yourself, it is not your partners responsibility to cater to your trust issues and to a degree I feel like it enable them. It goes to show them that the reason their partner is not cheating isn't because that they are genuinely loyal but because they just don't have the opportunity to.
Another reason I hear a lot for not wanting their partner to be friends with the opposite sex is because "it's embarrassing" or they think other people will think that their partner isn't loyal to the relationship. I feel you should know your partners intentions and if someone sees them being friends with someone of the opposite sex as flirting or being kind to people of the opposite sex as flirting that's their thing, at the end of the day you know what is true and most people do not assume that a man and a woman being friends is automatically flirty or romantic. I'd argue that the only people that really believe that are the ones that hold the idea that their partner should not be friends with the opposite sex.
I didn't include this in my title because I feel like less people would agree with this but it is something I've seen my classmates and some friends ask of their partner, but for them not be friends with people of the opposite gender plus anyone who is attracted to their gender (i.e., not wanting your boyfriend to be friends with any women regardless of their sexuality and not wanting them to be friends men who are attracted to men) regardless of what their partners actual sexuality is, this one honestly just baffled me on why someone would even ask this, like what are bisexual people supposed to do? Have no friends?
I understand that if someone is in a relationship like this and they comply to their partners wishes it is consensual and I have no business in someone else's relationship but I still find it weird and controlling.