Sexuality was an important issue i needed to set in Arramar. Not every medieval culture made anything except Hetrosexuality illegal. there are some cultures like Ancient Greece that accepted it and still managed to survive.
An example of this is in Arramar, Men and Women are still expected to marry the opposite sex. This is because the parents obviously want to continue their family line, which in turn would be impossible in Same-Sex Marriages.
In addition, Homosexual Marriages are not outlawed by many people see no point in them hence many simply staying in relationships.