Some people would deny that you can truly and fully love more than one person at a time. In their view, love represents a total devotion of one person—heart, soul, and body—to another, which implies that you can never love a second person without taking something away from the first. But this assumes that whatever you give when you love someone is limited or scarce, so that giving some (or more) to Jane or Joe means giving less to Janet or John. This may be true with some resources like time or money, but not as obviously true with respect to affection; after all, parents can have more than one child without loving any of them less, so why can't a person romantically love more than one person?
Another way to make such an argument is to claim that love is monogamous by definition—in other words, monogamy is an essential feature of true love, implying that polyamory is a contradiction in terms. (See Deborah Taj Anapol's great post on polyamory here.) But why? One could argue that by its very nature, loving somebody includes promising your affection exclusively, so the other person can reciprocate with confidence. But this assumes that both persons desire monogamy, which begs the question; naturally, monogamy-oriented people will desire monogamous relationships, but this doesn't explain the desire for monogamy itself! Of course, desiring a mongamous relationship doesn't need justification, but neither does a desire for any other type of relationship (including not being in a relationship at all, as Bella DePaulo emphasizes on her Living Single blog). But it seems hard to defend a essentially monogamous nature to love itself without first assuming that lovers want monogamy, which is circular reasoning.
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