Why not start where most romance novels start
Girl meets boy.. Upstanding good citizen
Girl meets other boy... More wild, free, sexually adventurous
Girl can't decide
Girl doesn't want to decide
Girl decides on both
Ensuing drama between boys
Girl convinces boys of love and uniqueness
All 3 live happily ever after
Most romance stuff doesn't work with married couples because I think, the idea of romance in marriage is lost and the concept of breaking out of marriage would be tough to break.
Starting with new love all around would be easier to sell I would think since at some time most of us have had to chose, wouldn't it be a romantic notion if we hadn't had to?