Sex in a relationship that, for one reason or another, has degenerated to near nothing. No one in this marriage is getting any except with themselves anymore. Happens after one or both loses interest or gets fed up with the joyless sex of the other. May take years of normal marriage to get to this point usually preceded by a period of sex that feels a lot like wanking anyway. Neither has the balls to admit it or the wherewithall to go out and just cheat on the other to get off. Also characterized to close friends - who couldn't help noticing the chill - as "mature" love.
Fred and Wendy practised same-sex marriage - after almost 20 years of marriage, their sex life was almost non-existent - if it hadn't been for the wanking they secretly indulged privately in, there wouldn't be any at all.