It seems that you and some others on this thread (e.g. @Swiftdove) seem to subscribe to the Mike Pence school of interaction with the opposite sex.
It’s completely alien to me. It was just never part of how I grew up and is not how I choose to live my life now. With my friends, it makes no difference whether they are men or women. And when I say “friends”, I mean close friends, people I’ve known over a period of several years or longer, people I would meet with one-to-one once or twice per month for a period of several hours, sometimes go somewhere together for a weekend.
Same goes for my fiancé, as I have said. He has this particularly close female friend. They went to school together, went to university together (although belonging to different colleges), and now live in the same city. Typically they meet every other weekend, visit a museum, see a movie, take a long walk, then catch up for a couple of hours over a coffee or lunch. Occasionally they meet on a weekday evening. When we’re married, I don’t intend for them to change their routine and I don’t expect to be invited along each time.
The point that nobody has picked up is how this applies to people attracted to the same sex or both (a category in which I don’t mind stating that I include myself). When gay/bisexual men and women are friends, they’re not constantly falling in love with everyone and having sex together. Indeed, if one is bisexual, can one have any friends at all? And, if you are heterosexual, how do you cope with having gay and bisexual friends?