Everyone has a right to his/her own opinions, so don't accuse those of us who don't find threesomes "desirable" as being judgemental or insecure. It's just a personal belief.
I admit, I can be a little jealous. But I just can't comprehend how anyone in a committed relationship would be ok with their partner having sex with someone else. I just don't get it. I cannot imagine my fiance having sex with some other girl -- right in front of my eyes! It makes me so sad! Granted, the thought of a threesome has crossed my mind once or twice, but I have never seriously considered doing it. I don't think I could deal with the aftermath or the consequences.
There are reasons for monogomous relationships. "monogomous" meaning you are only with one person -- sexually, emotionally, mentally, physically. If you want to sleep with more than one person, fine. But do it when you're "single" or not in a monogomous relationship. There are times for experimenting with sex, and times to settle down. If you're engaged or married, I don't see why you would engage in any sexual activity with anyone else. But hey, to each his own I guess.