I've never had caulerpa go se.xual on me in the last 12 years of reef keeping. Coral Keeper did, but he saw it fogging and took it out before it could do any damage. The reason his went se.xual was that he changed his yellowed PC lights to a more powerful metal halide. I believe the change in lighting and an increase in nutrient levels caused it's natural ability to go se.xual. JMO.
Chaeto is good, but when dealing with high nutrient levels, nothing can compete with how quickly caulerpa grows.