“So… I read a text that said [what you posted here]. It wasn’t intentional, I wasn’t looking for a text like that but now that I saw it, I can’t ignore it. What’s up with that?” And then see what he says from there.
There is a reasonable chance that this has nothing to do with anything actual sexual. Men have a tendency to repaint and exaggerate anything that could be even slightly homoerotic as an incident. They could have just bumped heads or fell on each other and my friends would have said something similar, especially when I was in my twenties. Just be careful and open on your approach, this isn’t the smoking gun you may think it is from your perspective.
Sit down with him and tell him what you saw and that he needs to explain himself for cheating on you.
Oof hopefully you get answers when you sit down and have a civil convo with him. One of my biggest fears is secretly marrying a bi or gay man. Happened to my one friend, everyone supported him and nobody even cared about her or her feelings. It’s messed up.
To begin with, ask yourself: do I really want to know this?
I have learned that many times, people ask questions they don’t want to know the answers. And this frustrates them more. If you are one of them, think it through.
But if you are like me and love to know the truth about things, then just write everything you wish to know (as a reference) and make a moment to bring this up. You know which is the best way to do it, you just have to loose the fear of doing it.
Super important : don’t judge him. Be ready to listen to him with an open mind. Let him express himself in a safe environment and work things out.
You got this!