I hope she's not a virgin. I hate all the "just so you know... I haven't been with anyone else" talks that crop up out of the blue for the male romances. It's like, who frakkin cares? Why must the love interests be virgins?
I would only see it as an issue for people if she was like Isabela and diddled just about anything with legs. That could be a huge turn off. But seeing as how Cassandra did not proposition Varric during the interrogation or get all handsy in DotS, I'm going to assume (dangerous, I know) that she is not like Isabela, so there would probably be no cause to worry about a weird rash or scale itch or whatever.
She is a grown woman, and I don't think it would be very believable for a woman her age to not have had sex.