It's understandable -- but never morally right -- to go outside of your marriage. And while not condoning it, I can understand the one-night-stand thing... maybe one has too much to drink, is feeling neglected and lonely, etc. But for a spouse to wake up daily, and make the conscious choice to continue to lie to their spouse and cheat on them, day after day . . .
I'm sorry, I just don't think it's right. And I cannot be "best friends" with such a person, who would do that to me.
