I could just say something like... I prefer MFM because I like watching my wife enjoy sex with other men because I love her... But I believe there is no one reason in the men's background to prefere MFM.
Every man is many-sided personality and when he is doing something the reasons of that are not so simple as he sometimes says of them, they are many-sided too. A bit of this, a bit of that and something else...
At first I want to be with my wife as close as possible during swinging. I just want to be close to watch her emotions, to hear her moans...Because of that I prefer MFM more than MFMF.
Secondly I am be curious and last time we have been looking for males who is be curious too. My wife likes to watch this. So I prefer MFM more than FMF.
We like double prenetration and MFM give us the possibility to realise that.
And so on.
As a rule I never share my fantasies like MFM or swinging with vanilla friends of mine but once I told my friend. Although that fact was an exception. He was very interesting in swinging and every time we met began the talk to this subject.