I agree. Players who prefer straight romances are not limited by a bisexual romance. 
However, if both Cullen and Scribbles are NPC's and straight, for example, that means that players who prefer straight romances will have one companion LI available, compared to players who prefer gay romances, who will have two companion romances. If Scribbles was gay and Cullen was straight, then players who prefer to play female characters who have two NPC LI's and only one companion LI, whereas players who prefer to play male characters who have three companions LI's and no NPC LI's.
Having the NPC LI's be bisexual means that straight Inquisitors, gay Inquisitors, female Inquisitors, and male Inquisitors all have the same ratio of LI: one companion, one NPC. Bisexual Inquisitors, of course, have all the fun.
That's why I think it's more likely for Cullen, if he is indeed a NPC, to be the male bisexual LI.
Oooooh, ok, now I see what you mean!

when you put it that way, then, yes, taking into account the mentality of companion > NPC I see what you mean about Bi NPCs being the most "fair." Since I'm personally fine with keeping an open mind about NPC LIs being awesome, I just wasn't looking at it that way.

but you may well be right, for sure.
Someone asked one of the Devs if romances would be race restricted and they didn't deny it. Just said "more info on this later" or something. We've never really had a LI who was as devout as Cullen, other than Sebastian? Alistair wasn't religious, so you could romance him as a Dwarf. Leliana had "unconventional" beliefs. I feel like Cullen would probably want his wife to be a fellow Andrastian, no? Maybe if you're a Dwarf or Qunari you have to convert first? 
Yeah, the only reason I suspect that race/class (or just in game major choices!) may also restrict romances is because the Devs have talked about the romances being more tailored to the character in question, or that's the impression I got anyway. I love a good "love conquers all!" or even a well written "rivalmance" but if they are going for "realism" sometimes things like the views in serious life-affecting issues (politics, religion, etc) does affect the development of romances, for some.
Okay, yep, there ya go; that's basically the major basis behind my fears.
Yeah, I understand. I'll be totally OK with being wrong, chica, trust me. If I'm just going on "what will make ALL THE CULLEN LOVERS HAPPY" I definitely want Cullen to be Bi and would be completely happy with it. So here's hoping I'm dead wrong!