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?
The guru (which was definitely not Laghima) that Aang spoke with was not saying that he is not allowed to love someone. He is saying that you need to be able to let the ones you love go if you need to.
Aang does this in the fight with Azula in the crystal caves when he realizes what it meant.
This is further backed up in LOK when Zaheer talks about Guru Laghima's teachings. He says you must "let go of your earthly tether" in order to be able to fly with air bending. After Zaheers partner Pi-Li dies he does exactly this and is able to fly away with Korra.