I'm trying really hard not to read this as "Cullen will not love your mage."
Settle down, Hannah, settle down! 
Remember the optimism? It was fun. I'm going to keep doing that.
Fingers crossed for bisexual Cullen, then!
If Cullen is bisexual, then Scribbles would probably be bisexual, too. Hmmm... LIST TIME.
The nonhumans cannot be on opposite ends of the sexuality spectrum, or else one gender will get only humans, so either both are the same sexuality or one one is bisexual. Also, the NPC's cannot be on opposites ends of the spectrum, or else one gender will get only one companion romance. So the best way to keep things equal (which I have to assume is a goal of the Dragon Age team) is to have the nonhumans be the same sexuality and the NPC's having the same sexuality.
On another hand, Dragon Age has done a lot of bisexual rogues. A lot. So I could see them avoiding that path, and if Sera is one of the romances, then she's either gay or straight. They said they were going for non-standard romance appearances, correct? So, I'll guess she's straight. I also believe the fairest way to handle the NPC romances is to make them bisexual. So, with that, here's Phate LI Asspull Table:
HOMOSEXUAL - Dorian, Cassandra
BISEXUAL - Cullen, Scribbles
STRAIGHT - Solas, Sera
Of course, I cold be completely wrong. In which case, here's Phate's Other IL Asspull Table:
HOMOSEXUAL - Dorian, Sera
BISEXUAL - Solas, Cassandra
STRAIGHT - Cullen, Scribbles
I rather prefer the first table, because it's not what you'd expect from looking at the party.