It's a strictly AU.
Aragorn is Gondor's king and must marry the son of King Thranduil, Legolas. But the king is already in a relationship with his captain, Boromir. When Legolas arrives to Minas Tirith, Aragorn informs him that he already has a lover and their marriage is one of connivance. Legolas just shrugs in response, saying that he held no feelings for the King either but he required that the king won't sire any children from anyone other than him, which the king accepts. but after their marriage, the two slowly fall in love with each other and Legolas finally conceives. however, about this time, someone tries to assassinate the prince by shooting an arrow at him while he was in the archery ranges.
It was in the Mirrormere site but since the site is down, i don't know where to find it.
there is a scene where Legolas and his guardian talk about marriage and stuff and the guardian warns the prince from sleeping with Aragorn before they were lawfully wed.