FFB
For a "friend with fringe benefits".

A close friend of either sex, who is very intimate, not in as commited a relationship as marriage but not excluding to have all the benefits of a marriage such as cohabiting, sharing confidences, etc.

Some explanation:

It started out between my friend and me, as not wanting to call our relationship just 'friendship'. Besides, we had our families, work and other commitments of social construct independent from and of each other. And we were too 'old' or 'old-fashined' to use the term boy friend/girlfriend, but not old fashioned enough to be prudish about a sexual relationship that happens when you meet someone 'special'. Hence we were 'friends' but having a variety of 'fringe benefits'!
We are FFB's who love to travel together frequently, but return to our families and all our other regular activites.