I think most animal studies demonstrate to us that we are probably not monogomous or designed to be, our purpose in life is to multiply as much as possible (ok I dont need the religious/scientific/gay theology here!
I just mean in 'simple terms'!!)
I think most of us seem to want something we cant have.
I have never had a relationship so I have never had your experience, but I have been in love/lust before and I have felt totally devoted to the person I had feelings about. But I guess after a while we want something different.
Thats why a lot of marriges break down, people cheat, and the couple who met in high school and who have only had sex with each other are not the norm. MOst people these days have multiple partners by the time they are 25!!
I think that if I was in a comitted relationship and I found out my partner was chatting to lads online, even with a no touching rule, id still be a bit upset about that!! More so as they kept it secret, Id nearly prefer to have been made aware of it, and hey I might even suggest having a 3some so long as we all consented, although youd then run the risk of being a spare wheel at the party, which can open another can of worms.