Liking someone who's in a relationship isn't wrong. Acting on those feelings would be, because you'd be intruding on someone else's happiness and a presumably happy relationship. But just having those feelings isn't wrong.
Having feelings is perfectly natural. Acting on those feelings is what decides if you're a good or bad person for doing so. At the risk of sounding controversial, I even believe that having sexual feelings for a child, or feeling so angry about something you want to murder someone, aren't wrong. However, I think paedophilia itself and murder are both wrong, and those who engage in either act should be punished to the fullest extent of the law (and yes, that includes the death penalty in countries where that still happens).
The difference is that the former are just thoughts and feelings, which are perfectly natural. The latter is acting on those feelings, which is down to a person's self-control, and not just down to nature. If you can't control yourself enough to not act on those feelings, then you shouldn't be out on the streets.
I realise that's gone completely off-topic, I just thought it would be a (somewhat) nice area for discussion in addition to the original topic at hand...