Check out Aurora Reservoir east of Denver. I used to sail there before moving to Indiana. No motor boats allowed. Nice sandy beach. Also check out
http://www.hobiefleet61.org/Boulder Res. has a few cats sailors, but I haven't sailed there, just visited for the annual kinetics race. I've sailed in Union Reservoir once, it was adequate. I think they had plans to improve the beach. Bigger water is available if you travel more. The best is Lake McConaughy, near Ogallala, NE. Also Lake Granby and Dillon Reservoir across the divide. So, yes, there are options. Plan on some driving.