It won't really come as a surprise to those who read my posts, but I don't really think the "traditional" nuclear family is anything but an outdated construct holding us back. I don't believe sex need to be as controversial as it is these days, or that it needs to be kept hidden, shameful and only between married couples.
I don't see anything wrong with having a sexual relationship with a friend, or multiple friends, without that changing your relationship. You don't have to be anything but friends. I also don't see how excluding yourself from multiple partners would necessarily improve a romantic relationship. I can see the appeal of sexual monogamy, but I don't believe it's necessary, or healthy for all people, in the long run. I also fail to see how sex needs to be connected with romance. Sure, sex will play a part in a romantic relationship, in some way or another, but it's just a part. A bigger part would be love, companionship, and communication. Seeing as all of those can be shared by others (in the case of love though, in other ways), I don't see why sex can't too.
I heard a saying once, that "a functional sex life is 5 % of a relationship, a dysfunctional one is 95% of a relationship", and I can agree with that. If the sex with your boyfriend/girlfriend doesn't work, things aren't going to work out well. If you can relieve your sexual tension elsewhere, that becomes less of an issue.
And this is only talking about monoamouros (or whatever the hell you English folk call it) relationships. A polyamouros relationship can work very well as well. We just have to get over outdated traditions and let our culture, our traditions and our society evolve as fast as we do, socially.