Good question mate. Love can wax and wane just like the moon. You can fall out of love very easily and suddenly. Not always for a logical reason either. A good friend of mine found out his wife of 10 years had been cheating on him for three years. No one could understand why he didn't leave her. We went out for a few drinks one night and I asked him that very question. 'I love her' is all he said. She ended up leaving him so obviously not mutual.
Cheating is the ultimate sign of love being a strictly, human creation. The same as marriage.
The faithful men and woman in this world have the ultimate willpower. They are going against the very fabric that they were born with. To mate and pro-create with the best suitable partner.
The best suitable partner always changes in the wild. Is cheating really that blasphemous? Or is it simply mammals giving in to nature's design?
My personal opinion: Scumbags cheat while married or in a committed relationship.