Bbarnsworth,
(As a previous "unicorn" I am going to refrain from certain comments and be a good girl here by moving on to your question.)
Mr. Femnewb and I have a single male gentleman we enjoy as a swing partner in MFM. We feel honored to have him as a play partner not only for who he is as a person, but as you bring up, also for the reasons he is in swing. He has a full time job, a parent who has health issues requiring his attention, and is currently starting a two-owner business in an unrelated field. He is charming, intelligent, well read, has a variety of interests indoors and outdoors, and several good friends. He was in the lifestyle previously with his ex-fiancee for 5 years. He was introduced to the lifestyle by an older couple (MFMs) years ago. He decided he did not have the time in his schedule, and the energy, in his life right now to devote to building and maintaining a dating relationship with a woman. He does not want to potentially destroy any good female friendships he has had for years by turning them into FWBs just to have sex in his world. For him, it just made sense to pop back into swing to find sexual play time and connections. While we are privey to some of his personal life we are all careful to not let his vanilla world see the foundation of his "friendship" with us. He is one of those rare single males who does not flaunt his interest to the world...a bennie for us.
And now having written this, I am not sure if this qualifies for your first category or your third.