Ultimate Girl Sin

As I was hanging out at Camp Shpayer this evening. [every Sunday night, I go to Josh Sh and Sigalle's to watch the obligatory TV- West Wing, Desperate Housewives and Grey's Anatomy] our discussion led as it always does to dating, love and marriage.

"If love were enough, I would be here with you now"~ Yes Sho, it was sad, and no I am not dead inside.

Women's friendships are sacred. I mean these two women on Grey's had the same guy donate sperm to them so they could have kids who were brother and sister. Because they were best friends. Don't worry Nacho, our kids can just be friends, no blood relation necessary.
So what is the ultimate sin? I think it can be described in one of two ways, but essentially it comes from the same root. And sadly I am guilty of both them. Which may explain why for much of my life I had equal if not more male friends than the female persuasion, that and my only having brothers.
1) Dating a guy you know your friends like. [It only took 2 years, but Shan now speaks to me!]And I am forever eternally sorry.
2) Stealing a guy from another female. This is almost as bad as dating your best friends ex behind her back or without permission.

If you find out the guy your in love with, has a wife, are you supposed to let him be? When he chooses the wife, is he allowed to be upset?

The more pressing question is, whether a relationship which starts off from one of the aforementioned sins is doomed to failure from the start?