VaughanJB
Scrappy VIP
I think it's the lying that would be the most hurtful thing, regardless of whether the relationship had become physical or not. If a partner has an interest in someone else, whether it's physical, emotional or both, then be honest and allow your partner, the person who you have no doubt professed to love, the choice of how they want to deal with that information.
For me, it would not matter what the nature of the affair was, the betrayal itself would be unforgivable.
Isn't the problem there that the first problem is usually a problem to communicate?