I cannot and will not speak for anyone but myself here. (at least not on purpose, please keep it in mind. I'm a jerk but for benevolent deity's sake, I try...)
I love my girlfriend. She makes me smile, and she makes me laugh. Sometimes she makes me think, sometimes she makes me question her sanity. She does for me what others don't, and indeed can't. I would like to think that I do the same for her. It becomes less about limiting yourself, less about stifling your love life, and more about realizing that nobody does for you what they do for you. If you need two girls/guys to satisfy you (as more than just a fantasy or an experiment) then are either of them truly any good for you?
I could never let anyone else get with my girlfriend. It completely undermines me. If she needs another person, then I am not good enough as I am, something about me either doesn't satisfy her or annoys her enough to look elsewhere. That, and I don't truly deserve her if I cannot make her happy. She shouldn't need anyone else for anything romantic if I am doing a good job.
Even on the primal level, normal intercourse to create a child can only be had by two people, unless you take turns. That is, in my thinking, why most people only have one mate, and why our society (at least where I come from) frowns upon polygamy. I have no problem with it, and have had many a daydream, but I just can't imagine caring for anyone else the way I care for my girl. Also, for every extra mate you take, that's one more fish you take from the metaphorical sea. Does one less person finds happiness because you took on an extra mate? I dont know, but I have no interest in finding out.