How does one know this to be true?
It shows an immense lack of respect to cheat on a faithful partner. No respect = No Love. Risking a relationship for your own selfish WANTS, means you didn't care enough about it in the first place. Otherwise, you would have made the CHOICE not to cheat.
If someone is going to cheat, they should leave their committed relationship FIRST.
Unless it's an open relationship, then things get all muddled ;)