Ooooh insy, I've changed my name to make this admission. Its the one bit of behaviour in my life that I'm not proud of (hence cowardly change of name).
I was friends for 10 years with someone who was 15 years older than me (we met through work). During that time she was with her partner who was 11 years younger than her. I became friends with both of them. I had a long term partner, and sometimes he would join us all for dinner, but most of the time it would be just me, cos invariably my friend would ask me to come over straight from work.
I never once considered that her partner was fanciable etc etc - I liked him, we got on, but he was off limits as far as I was concerned and I was in my own relationship.
My friend and her partner split up (never married, didnt have children).
About 8 months after they split up, my friend's ex partner rang me out of the blue and asked to see me. I wasnt sure exactly why, but was curious and went over.
To cut a very long story short, a few years on, he and I are now married with two children.
He says he always really, really liked me but would never attempt to do anything about it until his relationship with his partner was over.
As for me, well I had to choose between him and the friendship and I chose him.
I don't know what else to say really, except do what is best for you and your marriage.