My ex-wife had a similar rule. She did not care if I had sex with other women, however the only rule was no kissing. Her reasoning was that sex was purely physical and could be kept separate from feelings and the possibility of starting a relationship. Kissing on the other hand represented more of an emotional bond, which could result in an ongoing relationship (this was the threat).
I don't agree with this thinking, but do understand where she was coming from. Personally I feel that there is a LOT more involved in building a relationship than the kissing in the heat of sex.