Does 横恋慕 mean illicit love? means falling in love with someone who is already in a romantic relationship with someone else. However, according to the dictionary the translation would be illicit love. Can someone explain it to me? Why would it be illicit?

According to :

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means "falling in love with someone who is already married or in a relationship". So is _not always_ illicit; if you fell in love with someone who is already married, it would be considered an illicit love.