Interesting Q, Anirban Sen… not sure I can answer it entirely, though I will say: we are equipped to love more than one person at once without it detracting from our love for either person — consider the love for our child vs. partner vs. parent, for example. Or consider the love for more than one child, even.
If this is possible, one might suggest that it’s possible to love more than one partner — just in different ways, expressed at different moments.