I think it goes far deeper than sex which most do not realize. And it seems like people accept it more as they get older, I didn't understand when I was younger either.
Another issue is that I think is that many on the outside look at it as a way for bi guys to play with other guys. To be honest I don't really think they are wrong in many cases.
We are continually amazed at how many men we meet who say they are not into any contact or bi activity with another man. It is almost as if it is an expectation for men to state they are straight when going into an MFM situation.
Our first MFM was a surprise, with my wife seducing my lifelong best friend one evening at our house. There was no discussion regarding boundaries, likes, dislikes, and or anything else. But, once we were all naked, things just happened naturally. There was as much bi-activity as there was straight activity. I think initially, it took us all by surprise. There were a lot of discussions afterward between us, but we all agreed it was no big deal, and nobody was bothered by it.
The next man we hooked up with was at a hotel during a trip out of town. He hit on my wife on the dance floor, and she invited him to our room. He was young and a Marine on leave. He had a very ALPHA personality. When we walked into the room, he stated that he was 100% straight, and if I attempted any contact with him, it would not go well for me. I was fine, as this was all about my wife, and I had no desire to do anything with him. I figured the contact with my friend was a once-off fluke. He and my wife made out, and he ate her pussy while I prepared drinks and watched. My wife sucked his dick, and he told me to come over and for her to suck me off too. He unfastened my pants and pulled my cock out, then as my wife was sucking him, he took my cock in his mouth. He fucked my wife twice, and then he was all into me. I enjoyed it but was unsure how to reason his behavior with his opening statement. He had as much contact with me as he had with my wife.
Since then, over the last 16 years, we have found that 90% of the time in an MFM, there is bi-play between the guys. Almost all the men who feel the need to tell you how straight they are, end up initiating bi-play during the encounter. It is just natural. I don't think men seek out bi activity, but in private, they relax, and it happens. Ultimately, it is my wife they want. But, as most men soon learn, having your dick sucked or having your cock in a hot, tight hole is not as much about the gender of the other person but the feelings you receive and the pleasure.
We don't understand those who pass judgment on others for what they do in private.