[Grimnir]

If a man has a stag do, you know already that he is the type of character who puts his mates first and girlfriend second. Women who have hen nights believe in their friends first and man second.
So why the surprise if the man/woman cheats when you know beforehand you are not the number one love of their life? You are not even 2nd or 3rd or 4th.
For them it goes family>mates>lover.
If someone priority was lover>family>mates then they would not want to even have a stag do/hen night in the first place. Instead they would want to have maybe a romantic night with each other. To me, this couple seems a lot more in "love"
You have zero grounds for complaint if man/woman cheats on stag/hen night as you have already accepted your status as being second to the mates.