The way it started for us was a conversation about levels of attention. My wife wants much less attention than I do. She'd rather curl up in a book than with me after a certain amount of time. Our libidos are just as skewed.
She knew I needed more than she could give with out stress, and the best way to do that is to let me have relationships. They started out as just online friendships and sex play, but twice it developed into deeper feelings.
I would suggest starting with your needs and go from there. Don't just assume that adding someone in is going to meet them. Do you need someone to return your affection? Do you need more sex? Do you need someone to be romantic? Do you just need someone to talk to intimately?
Me: 41 straight male in a V with
Wife: Kay - mono female - married 19 years
LDR girlfriend: Susan - poly female - 4 year relationship
Stakes - very intimate friend
Ironically named Farmgirl - local interest