Actually both are partially romanceable, ie if you don't chose to pursue one of the main romances, you can have some romantic content with either one, and see what happens with it in the Epilogues. Diriel's outcome in particular is quite sensitive to what your PC elects to do romantically with him. Heck, if I were Obsidian I would have called it a full romance. (Insert the rolling on the floor laughing icon here). And yep, you have one particular poster, Kelenthial, to blame for it.
Mrs.Cake kindly gave me the authorization to use the portraits as defaults, so they will be the ones you'll see in the mod, and NWN1 ones will be the alternative