Well I hope that they don't discriminate either but this is a subject very near to my heart. I am a single mom for 20 years and have traveled with many children many, many, many times to Disney and we have always noticed that "families" are chosen for different things such as parades, events, ads, etc. My daughters, now grown, have just recently discussed this with me when we watched the new Disney travel CD. I must say that I truly have noticed this. One of my daughters just recently wrote to WDW to bring her observation to their attention. We have not heard back.
I hope I am proven to be wrong.