I have not had the experience of another man present during any kind of sex. My wife went through a period of wanting me to have oral and anal sex with a man, and I was (and still am, if we can find the right partner(s), actually interested. However, the potential partners weren't interested for the same (mostly exploratory) reasons we were.
I have to say the administration of the nozzle is very erotic, and the enema is good but more for me as a prelude to my wife using her strap-on.
Having a man exclusively would feel very...dangerous is the best word I can think of. Having my wife there would be very interesting, as I would be doing things at her direction. It sounds pretty domme/sub when I describe it that way, but it's more a matter of giving myself to her in a way that is not possible otherwise.
I will write another time about her first time using the strap-on - very kinky and very spiritual at the same time.
and a little confusing, but very satisfying.
As the man said, don't just stick your toe in the water, jump in and splash around.