Oh boy, this sure does happen to me... being in the full throes of NRE with the new BF, there are times when I am with him and I think to myself, "man, I don't ever want to be apart from him... I just want us to gobble each other up and be this close forever" and I get all woo-woo dreamy-eyed thinking about what sort of life we'd have together if we ran off somewhere and made babies, etc. etc....
But the thing is, I've been down that path before (well, not the making babies part, but the intense connection in a monogamously-oriented relationship that makes you want to throw caution to the wind) and I know where it would end... in a few months, I'd be back to missing my "older" BF, back to missing the variety of what two people (or potentially more) have to offer me, AND I would have spoiled what I had with both of them: with #1 because it would mean leaving him, and with #2 because I would be giving him the false idea that a monogamous relationship (which is something he'd want if it were possible) with me was a realistic option. Which it's not.
I admire them both for their patience in learning this lifestyle, which is new to all of us, because they care enough for me that they are willing to stretch the boundaries of what they thought a relationship "should" be like. Right now, I really do feel like the luckiest girl in the world, that I have two partners with whom I share interests and it seems 95% of values.
I ground myself by reminding myself that it's a wonderful thing that I have one partner with whom I am developing a solid foundation and we now have some shared history and are moving past the "giggles and rainbows" phase into something steadier, and another partner with whom I get to indulge in the heady rush of new love. And I know that once that new love mellows and steadies, if it lasts long enough, I will get past those urges to run off and live in the cloud castles forever-and-ever-and-a-day with my new partner, and come to appreciate a more solid love with them both