ES made some excellent points in his post.
We've never had a problem with a single man getting hard but have with men who are part of a couple.
When we are playing with single men it's very important to us that he gets as much out of the encounter as we do. We're very flexible in our play and always ask him what his biggest turn on is in a threesome...Does he like having the husband watch? Does he like having the husband participate? What do you enjoy? Since we enjoy it all we cater our playtime with singles as to what they enjoy this way we get what we enjoy and so do they.
With couples, we do the same thing (asking what turns them on) but, for the male half he's always going to be thinking about his wife/girlfriend and at times that can interfere with his own enjoyment of the encounter (i.e. getting hard). Haven't figured out a way to help with that and not really sure there would be anything that we could do or should do about it
There are so many variables when it comes to playing that sometimes no matter what you do, things just aren't going to work that night.
Teresa