I've never had that opportunity actually present itself, but I wouldn't have a problem with it.
An ex-wife was VERY active taking lovers, and I told her at one point if she wasn't going to stop, she might as well bring them home when she wanted... or needed a place to fuck. She said she would love for me to watch her, but somewhat surprisingly, she never did.
A current girlfriend has/had a guy she used to fuck regularly, and he has a lifetime pass to her pussy. She will fuck him anytime, anywhere, whether I'm there or not. He's the type of guy who will show up unannounced, immediately assert himself and establish dominance.
We did have a 3-way set up with him on one occasion, but it fell through at the last minute. In any case, he knows that even if I am staying with her on my visits back home, if he comes over, he will assume control of her pussy, mouth, and ass right away. If we can only work out the timing, we will share her, but he gets her attention first, and he'll be the one sleeping in our bed while I get the couch or spare bedroom.