If Cullen does end up the straight romance, it's not really the end of the world. You can always play the game again. Kind of the point anyway. These types of games have so many options that it will take many playthroughs to see most of the game.
I normally play females because I am one, but I actually played KOTOR twice (once female for Carth and once male for Bastila) and played a few male characters in SWTOR just to see the other side's romance options (imperial agent, jedi knight, jedi consular, and trooper).
I actually played ME1 twice and ME2 three times to see the Liara and Garrus romances (after doing the Kaiden one the first time) and then ME3 three times to see the Kaiden romance, the Garrus romance, and the Liara romance. All were with femShep.
I played DAO twice, too, with female Wardens (both human, one a warrior, the other a mage) telling myself I would only do the Alistair romance once. Did the Alistair romance with my warrior and married him. Replayed as my mage and forced myself to take Loghain so I didn't go after Alistair again (Alistair still became king), but I screwed myself out of a romance with Zevran and Leliana because I was trying to romance all 3 of them and did it wrong.
Played DA2 twice as well as a female Hawke. Romanced Anders the first time, tried to do Merrill the second and didn't like her so tried Fenris but didn't like him either, same with Isabela, so ended up with Anders a second time. Merrill was just too childish for me and I'm not a fan of Fenris's voice. Isabela I wasn't overly thrilled with, either. And Sebastian, while I liked him to a degree, was too much of a Chantry fanatic for me. If Nathaniel Howe had been an option, I would have gone after him.