Out of universe, the idea of the Jedi order as a monastic organization praising celibacy was a new concept introduced, if I remember correctly, in Star Wars Episode II: Attack of the Clones or around that time. Anyway, Luke made his marriage proposal to Mara in Timothy Zahn's Vision of the Future, published in September 1998, half a year before the release of Star Wars Episode I: The Phantom Menace. By this time, Jedi in the expanded universe were known to marry and have families. Forcing the divorce of a popular couple like Mara and Luke would had bean quite unpopular, so thing stayed that way.
In universe, it had to be retconned. All pre-Empire Jedi who were known to be married and/or having children were either given a special exception by the order or dismissed as rogue member who did it in secret or under disapproval from the order.
When Luke formed the New Jedi Order, it was years after his marriage and, as Chad mentioned in the comments, he felt love was an important part of the the light side. Many of the masters of the new order were also married and many had children. So, instead of establishing a retroactive ban on marriage, the prohibition was abolished and pointed out as one of the causes of the fall of the old order.
Excerpt form New Jedi Order, Growth and expansion section (emphasis mine)
Discoveries of Jedi relics, such as Jedi armor on Garn, lightsabers on Ossus, Asli Krimsan's holocron on Vjun and Arca Jeth's holocron on Arkania added to the Order's knowledge of ancient Jedi practices. Many of these would be implemented, such as pairing up Masters to apprentices and the formation of Jedi clans. Not all practices were assimilated, though, such as the Jedi prohibition on marriage, seeing as it was a marriage that was hidden (between Anakin Skywalker and Padmé Amidala) that destroyed the Old Jedi Order in the first place. Instead the new Order allowed marriage, even conducting Jedi ceremonies for Tionne and Kam Solusar, and later in 19 ABY, Luke Skywalker and Mara Jade. Many other Jedi, such as Kirana Ti, Corran Horn, Daye Azur-Jamin and Tyria Sarkin Tainer, married non Force-sensitives, with their children providing the next generation of Jedi trainees. The stark separation from family that was practiced by the Old Order would also be disregarded, with opportunities for employment and accommodation provided for trainees' families around the Academy.