Actually this is a very mature attitude and is beneficial for many relationships. However, I think the key word here is "mature". It takes a good bit of thought and overall understanding of human sexuality and relationships in general in order to make something like this work.
Unfortunately, many (most?) couples are existing in relationships where a warped understanding of love is the norm. When I say warped, I am speaking of the fact that most of what is passed of as love is actually a fear based possessiveness of another person. This fear, coupled with a sense of possession, becomes the root of jealousy. Thus, when I refer to the need for "maturity" it is simply an enlightened view of humanity that realizes that we can "own" nobody - love is only freely given.
If all relationships were built upon "true love" (e.g. a sacrificial love that only hopes for the best for the one loved), then there would be no rages of jealousy and there would be no need to sneak out and cheat behind the back of the one you (supposedly) love the most in the world. In reality, it's not the sex outside of marriage that is the problem but the pressure to remain monogamous and the subsequent failure to do so which in turn leads to deceptions and lies. It's the lies that kill - not the actual sex!
So, perhaps in time more people will become enlightened to what real love is and negotiated sexual relationships with others will be the norm. I like to think such broad scale enlightenment is possible....but, I actually doubt it will come to be. So the fear, cheating, lying, rages and murders will go on...and all because some other person had sex?????
