If Cullen really still is a virgin, then I hope that he chose not to do it because of work/beliefs/etc and not because he was too shy to talk to a female/male. Templars don't have to take a vow of chastity, so I can see him having sexual relations and why not? He is human after all.
In the end it doesn't really matter to me, I just want to know more about him in general. Where was he born? Who were his parents? Does he have siblings? Why did he join the order?
My theory has always been that if he is a virgin then it'll be because after all the sh!t he's gone through over the years he's just always had too much on his plate to do something about it. I mean, sure, he could go to a brothel or have a one night stand like all the DA2 Templars seemed to be doing, but he doesn't strike me as the type of man to have sex just for the pleasure, I think for him there would need to be at least feelings involved. Plus, he strikes me as the kind of man that wouldn't want to be a burden on another person, with all his head mess and the brainwashing he's had in the Order he's probably actively tried to avoid physical and romantic relationships, hell it's not like Templars have that much time to see their partners, anyway.
Interestingly, he says in DA2 if your Warden was an Amell (and this counts for Surana too) that he "never met her like again", which sounds like he goes for a certain type of person, someone that is skilled/impressive in ways beyond just physical appearance. So I don't think he'd be content to settle for a normal little house wife.
Perhaps he's just never met the right type person (since Amell/Surana, if you played as her, or at all, if not).