Cullen is straight. Despite mods, and despite literally a handful of lines that ended up deep in the game files, he hasn't been changed to bisexual. A lot of long time regulars of this thread watched the panel that David Gaider did back in 2014 at GaymerX Con. He was talking about a character that they wanted to make bisexual, but it "didn't fit." Once someone datamined the dialogue, most folks assume that Cullen was who David, Patrick Weekes and Karen Weekes were talking about.
Being bisexual isn't just about allowing both genders to romance the same person. I'm glad that they decided to make Cullen who he is instead of forcing him to be something else. For the record, I'm ambivalent when it comes to mods. It's not going to change BioWare's cannon. 
Here's my stance on that 'it doesn't fit the characters' thing - Saying that is deflection. Because they say 'it doesn't fit the character,' but that's based on biases of our world, not actually 'fitting' the character. Because Cassandra's romance? She has romantic notions, wanting to be wooed. We traditionally see that as a narrative for women who are seeking a relationship with men, but that's just because that's what we've always had in the past. I've played her romance, and if my Inquisitor were a woman, only dialogue referring to the Inquisitor's gender would really need to be changed. NOTHING about what happens is less fitting of those romantic notions if the Inquisitor is a woman, it's just not 'the traditional' narrative. Solas, Blackwall, and Cullen fall into traditionally masculine roles in their romances. The thing is THESE AREN'T GENDER-EXCLUSIVE ROLES. There are women who want to DO the wooing. There are men who want to be the emotional support for their lover. These things 'don't fit' the characters because they're using storylines that are traditionally designated 'masculine' and 'feminine' based on the parties involved, but these stories are not actually limited to those gender roles.
And the thing with Cullen? His story has MORE MEANING if he's a queer man, if he's bisexual. Addiction is a very real thing for queer people, this is a very common thing in the queer community. His story, if he's bisexual, gets to have an extra level of connection for queer people playing, because this is something that, statisically, they're likely to experience, either themselves or someone close to them. He VERY MUCH fits as a queer man, as a bisexual man. And there's another story in there, of someone whose sexuality has changed as he's gotten older and grown as a person - sexuality is not rigidly set, it's often fluid, falling on a spectrum. Given that he's got an arc about self-acceptance, accepting that he's bisexual is another part of that self-acceptance that really needs to be seen in media, in seeing someone who thought they were one thing and have over time realized that they're not.
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, just based on YouTube, haven't got there yet. Lord Shelden Willoughby Trevelyan VIII is still waiting for him. 
