Happy birthday in advance to you @jeaniepearl. More life to you. 

Hot Topic 
Infidelity should not be considered at any point. For the sake of this argument, I think the man/woman who is already married has the largest blame and should stop the act.

This is because a marriage is a commitment and should be honored by any means possible. Cheating is a breach of agreement. The single person on the other hand isn't commited to anyone and thus has lesser blame to carry