We have a lot of history together.  He was a boyfriend of mine in 1976, and I introduced him to my sister.  Because I didn't think we were that good a fit, and I thought that they would hit it off.  
She doesn't know that we used to sleep together before their marriage.  We didn't hook back up for 27 years.  From what I hear their sex life is virtually nonexistent, and they stay together for financial reasons having a home, acreage, vehicles in common.
As he puts it, "If your sister ever finds out, it's going to be pretty expensive."  Wasn't is Elizabeth Taylor who said something like, "You can't make a happy man stray."?
In the beginning  --  three years ago  --  I was divorced, celibate, and too self conscious about a weight gain to put myself back on the market.  But I felt comfortable and safe with him.  Because we had been together before.  He wanted me desperately which was/is very flattering.
Now, three years down the road, we have explored  our sexual fantasies together.  That's a very strong bond.
I have a mother and sister who do not return my phone calls.  I get an occasional note or email.  I wasn't invited to Thanksgiving, Christmas, or my niece's college graduation last year.  I get all my family information from the bil.  Including the times that family members come to visit but I am not included.  Maybe that's a lot of it.  There is a smug feeling of satisfaction knowing things they don't want me to know.  Passive-aggressive.
But he is kind to me, and he does care about me.  He is a good man in many ways.