Yes you can. SPOILERS AHOY!
However, the romance will be chaste, so no explicit lovemaking scenes. A friendly romance will have hm want you to become a Chantry sister so the two of you can have a chaste marriage, and a rivalmance will have him hint at a political marriage when he takes back Starkhaven. Also note that if you don't kill Anders during the Last Straw, Sebastian will leave the party.