I've heard the same, but I'm not sure how seriously to take the meme. I happened to read some Chrétien de Troyes chivalric romances recently (in translation), and the romantic relationships he depicts, while consistently idealized in character, sometimes conclude in marriage and aren't exclusively chaste. (Indeed, I'm told de Troyes introduced the character of Lancelot to the Arthurian canon, and we all know what he's famous for.)

Since de Troyes was writing in the 12th century, I'm inclined to take his work as at least somewhat authoritative.