For me I could never have a back up "nice guy" friend that's ready to step into the breach just in case. I had a guy friend who tried to undermine my relationship with my husband and pulled some crappy stunts so he had to go. Turned out that was his game, he did this with other women looking for an opportunity to pounce such as the relationship was having trouble offering an a shoulder to cry on.
I think in a relationship or marriage you should be completely committed.
I never had a back-up, how can a person be in a fully committed relationship if they have a back-up? I guess committed is the optional word.
Regardless, I couldn't fathom doing something like that to someone. Having a back-up, means the person already has plans to bounce...
Besides the stress of juggling two people - what they like don't like, scheduling conflicts, etc., it would just add to much drama to my life. Not to mention your playing around with the hearts of two other people (if they are expecting a committed relationship).
Not worth it!