While I appreciate that you are trying to be supportive of this configuration, your wording is a bit problematic for me because it implies ownership. “How wonderful of any husband/boyfriend to permit an ex or a stranger to share his sex hungry wife/girlfriend!” And I get that within the world at large, this is how people speak, and that it might just as well apply the other way around with a woman “allowing” for her husband/man to be shared.
Maybe this is why I rarely refer to my partner as my husband anymore — because it comes with so many connations that I don’t care for. We are partners; equals who each have our own personhood that we have chosen to link together. Yes, we have agreements about spending time with other people but he’s not “permitting” me do anything, nor I him. Sorry for being pedantic here, and yes, watching your partner with someone else can be a whole lot of fun. I wrote about it not that long ago. https://medium.com/@Ellesworld/she-comes-first-ba85c58aee8c