You know what - I think I may have given the wrong impression above AF, and I agree that if I was personally against this and only considering it to keep some waster dickwad, then it would be a completely wrong thing to do.
I would not do this so that he could sleep around with no consequences, just to keep him 'mine' or married to me.
I thought about it as a discussion point between the two of us. And you know what, I am not sure that he would want ME sleeping with other people, he may not be able to cope with that, and so may have to rethink the whole thing.
I, on the other hand, might quite like to sleep with other people 
I just wanted other peoples experiences / advice. I would not take this step without serious thought and working through the implications/rules.
SGB - thank you for your input (do you have to search for SGB on a daily basis in order to keep up?
)