Oh dear. David Gaider came right out and said that NPC romances would be lacking compared to party members, and they're still going ahead with it. XD
I think the perception of not having as much time with the character is the most difficult obstacle to making them "feel" like party-member romances, but we've already seen examples of the advisors appearing outside of Skyhold (e.g. Leliana at a party camp near the desert dig site). I wouldn't be surprised at all if they end up having nearly as many conversations as the party members.
Giving the NPC romances big, important roles in the game is another way to make them feel equivalent to the party members in terms of content, too - and given their inclusion in the Last Supper scene at the end of the trailer, Bioware are obviously promoting the idea that Cullen/Leliana/Scribe are significant. David said one of the challenges was ensuring that NPCs aren't just a romance and nothing else - but someone like Cullen appears to have a hefty amount of content relating to his Inquisition role even aside from his LI status.
Basically, I think David was suggesting that it's more difficult to make non-companion romances as fulfilling for some players as those with party members, considering the challenges of not being able to run around with them in the game world, and the likelihood that non-party characters will have less content overall. I don't think that precludes them from doing NPC romances, though - it just means they need to be careful, I guess.
Getting back to DHMG, I think it's more likely they he'd be either an m/m and m/f option, or exclusively m/m. I don't think people who enjoy same-sex romances have to worry about that content being "marginalised" in the form of NPC romances, and I'm pretty confident at least one party member will be an m/m option of some sort. Probably two.