Maybe he doesn't want to marry a woman and that's one of the things he wants to change?
But there's STILL the issue of children and the heir. It's somewhat irresponsible for Dorian not to have children as he is th e heir apparent of the House. Frankly, given the attitudes in Thedas and Tevinter specifically, waht exactly is stopping Dorian from having a marriage of convenicence and having a male companion....
Orlais doesn't seem to care (Leliana will actually be introduced in the Winter Ball as the mistress of the Queen of Ferelden if she was in a relationship with the warden and the warden married Alistair.