When we've looked back at our experiences we also noted that about 50% of the men had a problem keeping it up. We think some of it has to do with another man in the room, some just can't perform under pressure, some it was medical conditions.
For instance, one husband of a regular play couple of ours couldn't ever keep it up. At his annual physical his doc said he had high cholesterol and put him on Lipitor. From then on he'd wear Mrs. WS out!
Another husband has no problem keeping it up with Mrs. WS, but takes forever to cum. With his wife he comes in a reasonable period of time. Another form of performance anxiety.
And some just plain have performance anxiety where they psyche themselves out before they ever start. They know their wife really well and what to do to make her happy, but they get all flustered and worried about how they'll perform with another woman and then because of their worry it's over before they even begin.
There could be other reasons, too... but those are the most common we've identified. Most of them go away after a couple of times playing with them and they become more comfortable with Mrs. WS or us in general.
Mr. WS