Because some tend to think with things other than their brains......
Why do humans commit adultery at the risk of hurting all involved?
Because they are selfish. If they want out they should get out with both feet not just one. That is not exactly unconditional love.
When someone else turns you on more than your spouse, you make a choice to work harder on your relationship and you fight the urge as you would expect your spouse to, or you tell your spouse you feel as though you can no longer stay faithful, and should leave before you are.