Certainly more rare to use that title and not be monogomous yet.. but not unheard of or quite as rare as you suggest. Though I usually find the pattern goes the other way, people who are BF/GF when they first adopt the title it coincides with monogomy. However once they have been together a long time if they want to see other people as they begin to question the relationship they may do so without breaking up... "We are on a brake [from monogomy]" is usually how i hear it described and it seems to be somewhat common.