Hmmm... *pulls up the comfy recliner and curls up for a good think*
There is no simple way to answer this one. In my little slice of time as a thinkng and loving member of the human race, I have loved a small number of times. "In love?" That's an even smaller number. Love overlaps easily for me, however. I find affection for people in places and for reasons that confuse other people and partners. I enjoy finding what it is in all sorts of people that is worthy of being loved. I don't believe that by giving love I take it away from anyone else, as I do not believe we have a finite amount of it to offer by any means.
"In love," on the other hand... Being in love, to me, is a very singular devotion. It is this way because to fall in love for me is to want to give my life to my partner and that I only have one of. I cannot, in both a vanilla and in a BDSM sense, serve more than a single Master. How could I? If the time were to come when both called me to come to them, when both asked of me to be of service, I could only go to one and that one would be the one that I serve in the depths of my being. I can understand and entertain poly, and could even make it through a life practicing it, I suppose, assuming that the depth of the truth would never be probled far enough to reveal that only one or neither of my loves were the stuff of my ultimate affection.
I believe in the one.