I think many single males think they are up for a threesome when in fact they are not. The numerous posts on this board about erectile dysfunction at the critical moment attest to that. They think it's a great fantasy, but the reality is different.
Then there is the whole homophobic thing, getting their penis too close to hubbies. It's like a spoofed motivational poster I once saw with a MFM on it. The caption was "Teamwork... It's only gay if your balls touch." Well, if they don't touch the DP aint' gonna' happen.
All I can say though is, "I've done it... and it was FABULOUS!"
We have a single guy friend who was invited to a gang bang with two women and allot of guys. Only him and one other guy showed-up, but many RSVP'd.
I think another reason is that many are married posing as single and they can't get away from the wife for the date. Probably the #1 reason their single guys stood them up.
Mr. WS