I don't think there needs to be a story reason for it, he just simply likes women, same as gay men like males and lesbians like females. I'm curious about why Solas and Cullen won't romance certain races though, there's definitely a story reason behind that... for Solas, at least.
If he was Dalish I would understand why he wouldn't enter into a romance with the other races because of the keeping Dalish culture and elves pure-thing. And as far as I'm aware, we don't have much to go on about Solas' character but maybe he just prefers his own race and opposite gender? which I realise is an unsatisfactory proposal but the little information that we have of him isn't enough to say "yeah, Solas isn't attracted to (insert) so that's why (insert) he prefers female dalish elves.
As for Cullen. I'm pretty sure that it has been stated somewhere already that he's just not into Kossiths and dwarves. Which is fine considering that not everyone is going to share the same cup of preferences as each other.
When the romances were all revealed (when I was lurking on the forum) I was quite disappointed by the amount of people who were bitterly (and vocally) disappointed that they weren't able to romance x but got z instead. And I feel that the choices that the devs made when it came to allocating which of the characters were to be romanceable is quite realistic. In real-life, you see this everyday. You may like someone, so very very much (and we've all been there) but there's no guarantee that that person will like you back because of many reasons.
All in all, I'm just grateful that we have choices and a chance to have a lot of replay value for the game as sometimes with some games, I find that once I've completed the game, the reply value goes down the drain.
Now...as for Cullen's outfit....now that's nice. I particularly like the lionhead helmet.
I wonder if it symbolises anything? If it does...