Iin2ask your partner permission to sleep with someone you've just met?
I've never done it myself because I don't think my partner would be happy and - a bigger factor for me - I feel any friends with me would consider it the sort of thing only slags do.
However, I have met a few guys when out clubbing with friends who I would have considered sleeping with but then felt like I had to push away for the reasons above/because I didn't have protection on me.
I would love to have a Berger-Blomkvist situation, but I know it'll never happen as the majority of people I know are really old-fashioned and/or particular about monogamy and have very tight boundaries as to what they consider cheating.