Oh, gee, I'm here on a fantasy thread talking about sex. How out of character for me.
Well, I have to be frank, when I love someone I would NOT want to see them touching another woman. Perhaps it's just an incurable lack of self esteem on my part that makes me feel that way, but I think it would hurt me to see it. Now I would consider having sex with another woman while he watched. It's never happened, in fact, I've never had sex with a woman but I would absolutely consider it.
I've never had casual sex per se (I've always been at least somewhat attached), but if I were with a man I had no romantic attachment to then yes, I could probably swing that way with him.
And for the record, I do have/have had fantasies that I have no desire to act out in real life.