Yes, aspiring monogamists will fit your bill of people who "want to be off the site in 12 months" or sooner. That's one segment of your users, but it really isn't everyone by a long shot.
Plenty of users are signing up for the chance to meet ("get to know") a steady stream of people. We don't stigmatize people who subscribe to Netflix for many years so that they can keep watching different movies and shows. There's some segment of the dating-site world that has more of a Netflix model in mind.