Guys and girls being best friends can be fun, but it doesn’t always end up working out, especially if one person is in a relationship.
Jealously is a big problem in these kinds of friendships. One person in the friendship can be in a relationship or one person in the friendship is friends with someone the other person does not like or approve of, which can cause problems/jealousy.
Personally, guys or girls in long-term relationships shouldn’t be best friends with someone of the opposite gender. For example, girlfriends will tend to feel like they are in some sort of competition with their boyfriend’s best friend or vice-versa. Relationships are not meant for sharing your partner with others, such as when you want to hang out with your boyfriend but he has plans with his best friend.
Best friends also tend to become too attached to each other. It’s not good in the long run because most of the time someone in the friendship ends up falling in love with one another. When your love becomes noticeable, sooner or later you will have to tell your best friend and there is a chance they might feel the same way towards you but not always. At the end of the day, your friendship will get ruined because who wants to be in an awkward friendship?