I have done this a few times with couples my age (middle-aged) where it was the husband's fantasy. In all cases the husbands watched us and sometimes indicated they wanted to play later, but usually couldn't get hard to do the deed once I had finished. I do remember sharing a spa bath with a couple afterwards (he did get hard and he did have sex with her, which was hot for me 'cause she was full of my come). He was was also turned-on by fucking her pussy full of my come. In the spa he asked her if my size felt better than his but she was very diplomatic with her answer.
In all cases it's been the husband wating a well-hung man to have sex with his wife while he watches, and in all cases it's been mandatory that we have condom-free sex, even though they have no idea of my sexual past. In one instance the husband got jealous that his wife and I had sex (she played hard to get so I seduced her with some French), and he got the hump and watched cricket on television instead. In one case the wife seemed quite eager but then bailed on me and locked herself in the bathroom. In most cases (except the couple with the spa bath), nerves got the better of them and the husbands ended up drinking to get themselves through the ordeal, which is why they couldn't perform. Another couple wanted me to have sex with her while he watched us, and we all got on well together so it was one of the best for me.
I didn;t apologise to the husband who watched cricket, but I did tell him that reality is different to a fantasy.