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Yes!
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Like, in what context? Like as "fancy them" even though they are in a relationship or like as in "they like men" who are in relationships, a particular turn on if you will.
I think it's pretty normal for someone of either gender to like someone who is already in a relationship, a lot of us can admit we have had a little crush on taken or married men/woman, but acting on these feelings is a whole another ball game.
On the flip side of the coin, going after married people, in particular, is something completely different. Not every woman (or man) go after married folk and quite simply put, those who do are homewrecking idiots with zero morales.