I'm going to tackle this from an unexpected angle, adrift.
It looks to me as though you're making some assumptions, of which you seem to be unaware.
I think you're assuming that if your H found a side interest, the lady would be a decent sort who would play fair, accept her role as 'the chick on the side', enjoy a transient fling, feel no competitiveness towards you at all, never want your hubby around on the big holidays, and not try to push you out of your home and take over the marriage for herself.
Now, ask yourself: what kind of woman knowingly gets involved with another woman's husband in the first place?
Two kinds, mainly.
Kind One: lonely [or young, or both] romantics who get in over their head - usually these women are the approached, not the approachers. They usually pay a terrible price for the involvement... and often feel guilty as heck about what they've done to another woman, afterwards. You gonna respect your husband for messing with some kid's head like that?
Kind Two [or if you prefer, Unkind One]: Predators. Of all ages. I knew a twentysomething who got really sick kicks out of destroying other womens' marriages. I also knew a fiftysomething who had the same nest-soiling urges. And I knew a couple of gals in different companies I worked for who used the old 'sleep with the boss and get preggers' stunt to pry executives away from their wives and pre-existing kids.
You want your husband messing around with creatures like that? Because that's what's out there, and they'll home in on him like barracuda, if he's sending signals.
In re you being the cheata, ask yourself how you would insure that the guy you pick to mess around with wouldn't stalk you once you decided the game was over, or go irrational and try to hurt your husband in order to get you away from him.
I'm just saying... this kind of thing is messy, messy, messy. Very few Open Marriages last once they've been opened, and very few people can actually accept [and forgive] in practice what appeals to them in theory. Sometimes one partner discovers they really don't want the marriage to be open, after all - and the other one says: the heck with you, I'm enjoying this...
Who really wants to invite that kind of pain into their lives?
I am not making the argument from morals, here, and it's for a reason: I think the argument from ethics and abnormal psychology is probably what underlies the moral restrictions in the first place.
Spoken as one who lost her ex to cheating [well, actually I kicked him to the curb when I found out], but also as one who has received a few invitations from would-be cheating hubbies.
