friendspouse

A friendspouse is either:

1. a friend with whom you are so close that you are practically married;
2. a spouse who is also your best friend.

It could also be used as an abbreviated form of "friend or spouse".

1. People are always saying that Jordan and I are like an old married couple. We can finish each other's sentences, but we don't have sex. I think of him as my friendspouse.
2. Lisa and I were best friends before we got married, so now we're friendspouses.