Oh crikey, this is a really difficult one and frankly you will feel bad which everway you go.
If you don't invite them, given you have to disinvite one pair, when the relationship is exposed, it will be obvious you knew and didn't tell the innocent parties and they be devastated and feel betrayed. It will be easy to put two and two together.
Part of me wants you to tell gym bunny to decline the invitation if she knows whats good for her but again you hurt the innocent partner.
However, the conundrum is really whether you expose what you know not your wedding. Sadly and I know you might not like this but your wedding, your feelings, are secondary here.
You have two options, tell the innocent partners or keep quiet and stay out of it. If you warn off guilty partners from coming to the wedding, when innocent parties discover their betrayal they will realise you knew and said nothing.
So either invites stand and you zip it, no matter how uncomfortable it is and do not get involved, or you spill the beans now.
No one but you can decide this and I have been in this exact situation with friends. I will be honest, I was very close to one innocent partner and having been cheated on myself, I spilled the beans. It devastated my friendship group because there were those who it became obvious had been aware well before me and had said nothing. There were also recriminations with me being labelled a troublemaker to try cast doubt on my revelations..
But, and here is the real humdinger, it turned out my friend, "the innocent one", was not as innocent as I believed and she was responsible for the state of the relationship that made her partner look elsewhere. I was stunned and the truth is you never know what goes on behind closed doors.
So speak up or button it, only you have the facts and know the dynamics, there is no "one answer fits all". However there are usually innocent victims.
So sorry you are in this situation as its so unsettling but your wedding, sadly is not the issue at hand.