Lately, I like the idea of a threesome. MFM.
By the time my wife, Becky, and her first poly boyfriend (Ben) broke up a couple of years ago, I had become comfortable enough with the idea of her having sex with other men to offer her the opportunity to fulfill her long time fantasy of an MFM threesome. We had opened up our mono marriage so that she could explore here resurgent interest in an old college boyfriend. They were together a couple of years, and he even lived with us for some time, but eventually - for reasons still not clear - he felt the need to move on. While he lived with us, I had overheard them having sex several times, and "caught" them making out a few times - but sex with him was always behind closed doors, and he had no interest in a threesome (to my wife's disappointment). But as a result of the exposure to her intimate relationship with him, I had come from originally being mortified at the idea of her having sex with another man (when she asked me to open our marriage), to being comfortable enough to offer her a "swinger style" anonymous MFM threesome - to fulfill her favorite fantasy - and to help her get over her heartbreak. She had asked for us to go poly, and now he had left, while I still had a long distance partner and local fwb/casual gf (and still do).
So, we signed up for a swinger site, and went through the screening/vetting process, met Sam for coffee, then drinks, and finally the main event, which went well for all concerned. And Sam ended up becoming Becky's new poly boyfriend. They are still together, and we continued to have occasional MFM threesomes - every couple of months or so - Becky is always up for it.
(We - both guys - are hetero, and don't interact other than inadvertent touch. We are both comfortable with the scenario and don't freak about casual contact. One obviously cannot be homophobic and have an MFM threesome).
Obviously, one cannot assume that someone you meet on a swinger site will end up becoming a poly partner - that was just mostly luck. Although, some times swinger partners (probably especially single guys) might be willing to become "regular play partners", but again one never knows that going in to a scene, or whether they would work out at all. Still, if you and your wife want the experience of a MFM threesome, there are obviously opportunities on the swinger sites. Make sure you educate yourself first - and make sure your ad makes it plain that you are not a "cuck/hotwife" couple looking for a "bull" (unless that kink appeals to you). Vet and screen carefully - meet in a public for coffee or drinks first.
From a poly perspective, MFM V relationships are not uncommon in the poly world (woman with two male partners, where the 2 males are not romantically/sexually connected) - but not all such relationships would include MFM threesomes in bed. Some obviously, but probably not most.
Edit addition:
And, in regard to the topic of the original post. From my time in this forum and what I have read here and elsewhere, it does seem that MFM V's do significantly outnumber FMF V's - public fantasy perception not withstanding.