This is the age old debate. Reddit seems to be pretty adamant about the fact they can be friends while IRL has a pretty solid split and any given person could answer this question any which way.

Also, for the purposes of this post, being a close friend includes things like one on one hangouts and physical affection that would otherwise be normal between same sex friends and other things like complementing outfits etc etc.

Here's my thinking as to why they generally can't be friends. Essentially, it boils down to societal norms. There are just certain things you do at certain times and also certain things you do with different people. Like for example, if a woman is sitting at a cafe scrolling on her laptop, walking at university or at a park, or is out shopping and a man just introduces himself, she will assume, and usually be right about, that he is coming with sexual intentions. She would not assume this of another woman in almost any case, right?

Secondly, once a straight person is in a relationship, it can often make it look suspicious when they hang out with people of the opposite sex. Hanging out alone together gives a public appearance of inappropriateness generally speaking.

Also, another thing is that complementing your opposite sex friend is seen as sexual generally speaking. Let's say that you're a guy whose genuinely friends with an objectively attractive woman wearing a tanktop and daisy dukes or something revealing like that. If you say "I love your outfit", she will likely take it as sexual interest. That's just the reality.

To further illustrate this, let's say you have a woman who secretly is into said woman and said man is not into her at all. If both give the same complement, she will assume it's the man who's interested even though she's wrong in this case. That is just the strength of our societal standards.

Or better yet, try replacing the outfit compliments above with a complement like "I love your eyes" or "I love your hair today." Assuming that nobody in this scenario has revealed their orientations, if a man or woman give this compliment to the same woman, she's going to assume the woman is being friendly and the man is making a move.

Of course, there's differences in physical affection too. If a woman hugs another woman for a good minute, that is seen as friendly but if a man were to do it, it would (usually rightfully) be seen as an indicator of sexual interest.

The point is this: A lot of things that each genders do in their same sex friendships would be seen as "making a move" in an opposite sex friendships. Especially if we look at the stuff normal in female friendships. If a man were to try such things with his female friend, those are (again, usually rightfully) seen as making a move.

Also, I think most counterarguments are weaker than most make it out to be. First off, the bisexuals exist argument. First off, the two people in this scenario are straight. "What if I was not straight" is not a valid argument if you are straight and if you're trying to justify your friendship, you should be able to do so in reality and not need to resort to different hypotheticals. You should be able to justify your opposite sex friendship in this reality where you are straight. And the reality is this: a bisexual person's inability to make a friend group with people of sexes they're not attracted to does not preclude that a straight person is able to and thus can be held accountable to that standard. If you're straight, you can do this and are liable to be obligated to do so.

Secondly, different orientations have had different social norms forever and there's no reason this needs to be a problem. For example, it's normal for gay men to be invited to all woman gatherings but straight men generally are not. So, just based off of that, it is totally fine for differing orientations to have differing social norms.

As far as bisexual individuals go, there's no rule that a societal norm regarding straight people to be perfectly analagous and translatable to bisexual people. They are differing orientations and can have different rules societally as far as friendships goes. And as I said before, a bisexual person's inability to make a friend group with people of sexes they're not attracted to does not preclude that a straight person is able to and thus can be held accountable to that standard.