Many straight men aren't afraid of expressing themselves emotionally. These are men who are confident in their masculinity. A kiss isn't necessarily sexual.
What? Why in the hell would you want to kiss another bloke on the mouth as a straight man?
what is it if not sexual?
For fun? A greeting? Friendly gesture? Show of appreciation and respect?

What are two straight men trying to accomplish by doing that when you can shake hands instead?
When it comes to celebs and actors , its easy to see they did it as a PR-stunt (for upcoming shit and whatnot), but if youre not an actor viewed by millions? Why would you want to do such a thing if it aint sexual?