Aside from the fact that the majority of SMs on these sites aren't truly single, you are looking for a very specific type of play. I would guess that most truly single males are here for casual sex & fantasy fulfillment, not a poly relationship.
That said, I can only speak from my experience when I was an SM from about 2007-2015. Over that time, I had several "relationships" that bordered on poly where husbands allowed me to spend a lot of time with their wives, taking her to a club, out on dates, overnight stays etc. While I enjoyed the regularity of these situations, there were some drawbacks. One of the hotwives eventually wanted me to be exclusive to her. That was the end of that. One of the women caught the feels and left her husband, I ended that one too. Another decided to leave her husband (unrelated to me) but wanted me to stop seeing other women and be monogamous with her in a vanilla relationship.
I did have one long-term situation that was drama-free and a lot of fun. The husband would joke that I was her "second husband" and wondered why I never came over to help mow the lawn. lol
Ultimately though, these relationships were pretty unfulfilling for me. The sex was great and I lived out many, many fantasies but at the end of the day, there was no true intimacy as that is reserved for you, her husband. When our time together was over, the two of you enjoyed reclaiming sex and then back to your everyday routine together and I went home alone.
Fortunately for me, I met rabbit in early 2015 and we have been together now for 9 years and we get to enjoy this LS together.
I am sure you will find some guys who would enjoy what you are offering, this would been right up my alley "back in the day". ;^D Just realize that it is a small niche of available SMs and probably as rare as the proverbial unicorn SF.
Good luck!
~Scamp