The most obvious problem is that you may be devoting resources to the other person—especially time—that your committed partner expects from you.
But let's imagine that this doesn't happen; that is, you manage to engage in the new relationship without neglecting your partner in terms of presence (say, by corresponding with your secret someone by email at work).
(And if your partner is not aware of the other relationship, then you've brought deception into the mix, either through silence, hiding, and sneaking around, or by outright lying.) Of course, your significant other may not value monogamy, in which case presumably you can be open about your other relationship.
Another possibility is that your partner is no longer emotionally committed to the relationship, which is maintained for other reasons, such as children, finances, cultural or religious factors, and so forth.
(See Deborah Taj Anapol's great post on polyamory here.) But why?
Of course, he or she may agree with it (or have reconciled to it), and there is a sense in which you can accept this as justification.And for the time being, we're not going to worry about defining emotional infidelity—does flirting count, or what about emailing or texting throughout the day, and so on—that's a topic for another day and another post (or perhaps another blogger! We're talking about being in love or falling in love with another person, which I think most people would consider to represent emotional infidelity (or, at least, is a good sign of it).Some people would deny that you can truly and fully love more than one person at a time.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.This is tough—how often do we find true love in the first place?And how frustrating is it when we find it but it comes at such an inopportune time, such as when you're in another relationship?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 De Paulo emphasizes on her Living Single blog).This is reminiscient of what I wrote in my post regarding inadequacy: it's one thing to respect the other person's choice, but it's another to hang too much weight on that when you feel it's not the best choice for him or her.Do you really want this person you love—more than your committed partner—to settle for second place, in your day-to-day life if not your heart?