In season five and six i'd have agreed with you there. By season seven however Dawn was already a much more mature individual and after another year or two had passed i seriously doubt that there would be much difference in maturity levels between Xander and Dawn.
This topic keeps getting me to thinking about my relationship with my "step-sister", Tracey. My mum and her dad where together for a good few years and because of that we spent a fair amount of time together as kids. Back then i never would have viewed her as anything other than my step-dad's daughter, although i still never looked at her as a sister.
Years later, after our parents had split up i ended up going to college with Tracey. She is three years younger than me but because i decided to get a job after school but then went back into full time education i ended up in the same year as her. Boy, was she NOT the same girl that used to bug me as a kid.
Our relationship was entirely different and on a whole new level. She was now a young woman and we had things in common that weren't there before. We never dated but that wasn't because we didn't want to or felt like we couldn't, but because we were both always with somebody else.
The point being that people change as they get older and relationships change with them. Just as a sexual relationship can become a friendship, so can a friendship become something more. Ultimately in years to come there is no saying what could develop between Xander and Dawn.