Hello How - and welcome !
I think Imaginary's post was par excellence ! Covered it all well.
The compersion thing is huge
If you can cling to that you'll make everyone around you have a nicer life.
And really - if you think about it, if - as Imaginary pointed out - you have no negative concerns about this guy - what harm is a little "romp" ? It won't change what the 3 of you have together. Unless you make it change
The only concern here of course is the fact that he's got a mate at home and how is THAT going to play out ? Last thing anyone needs is more drama in their life. Something to think about.