When Varric is describing Hawke's exploits he jokes that Cassandra seems to be showing a bit of hero worship. I think how she views anyone, not just a mage, will depend on their actions. Regaylan was a good mage who managed to overcome her hatred for mages by the way he behaved. Whether she felt anything more than platonic affection is irrelevant. The fact was he had overcome her prejudices against him.
Rhys and Evangeline got together because again both of them were essentially good people and ultimately came to recognise that despite their culture and inbuilt prejudices.
Wynne had a relationship with a Templar. That is how Rhys came about. There is no way of knowing whether it was a romance over a period of time or just a brief fling but clearly both sides were able to overcome their inbuilt prejudices and the rules against such a relationship. It was definitely a consenting sexual encounter.
So I think your class will have less effect on your romance potential than how you conduct yourself. Cassandra is not the sort of person to engage in a quick fling. We are told that deep down she is a romantic. So you are going to have to convince her that your feelings are genuine and that you are worth her committing herself to for the long haul. I'm guessing that once you do, she will be an utterly faithful and devoted lover.