It seems like the guru's rule of no Earthly attachments (or not loving anyone) was only valid at the end of book 2. Past avatars, like Roku and Kuruk, were both fully capable of entering the Avatar State while being in love. Aang himself masters the Avatar State while loving Katara, as shown in the Legend of Korra and even more so in the comics. The guru clearly said the Avatar State is impossible as long as there is an Earthly attachment. Was he really saying that the Avatar needs to learn how to "switch off" love temporarily in order to enter the Avatar State?
Earthly attachment is not about loving anyone; actually loving someone is not earthly at all.
Earthly attachments are about material attachments: Selfishness, pride, etc.