It has clearly, understandably, shocked and upset you 💐
First thing though, do not speak to his ‘friend’ about it. You don’t like like the guy anyway, he has form as a cheat and a liar. No good will come of talking to him.
Secondly, this absolutely does not have to be the end of your marriage and you do not have to make that decision immediately either.
I would say it doesn’t matter if the other person is male or female if a partner has been having an emotional affair or actual sex with someone else. ThIs, is not that.
It’s something everyone else who doesn’t do it struggles to understand the mentality of, totally straight men doing this stupid greeting/post match man grope. One of my male friends is a really lovely bloke, but part of that whole rugby scene that act like this. He is straight, nothing repressed, no interest in men sexually. I know him inside out, he would definitely tell me if this wasn’t the case. Absolutely. I don’t understand why they do it, neither does he. But it’s not sexual.
I have another male friend too who has very definitely opened my eyes to things I would would have previously viewed quite differently and been adamant about. I am now far less rigid in my views that doing x automatically means y. I can understand that people who haven’t had these discussions might still think differently, I would have.
It’s understandable that you’re confused about what this means, but I honestly believe that it’s could be exactly what your DH has said it is. An unexplainable thing this group of blokes (and many other groups of blokes) does, in a pub/locker room way. Nothing more. Nothing ‘cheating/affair’ about it any way.
Imo it is not the same, at all, as discovering your DH has been unfaithful/cheating on you, emotionally and/or sexually. Secretly meeting up to ‘be together’ or have sex. This is an odd thing done in a public place amongst mates.
However, you need to give yourself time to process it and decide what it means to you & in your marriage.
I would stay home this weekend, get your Christmas tree etc just process it in your background thoughts.
All I can say is that I wouldn't be throwing away a 17 year relationship, with a fabulous husband & dad, who has always made me feel very secure and treasured because of this stupid behaviour. BUT YOU have every right to do what is right for you 🌷