I would have answered him with something that I wanted to do to him... not something that I wanted him to do to me.
You'll have to use your imagination while thinking about past sexual encounters and what took place during those times (whether or not those encounters took place with the man you're talking dirty to doesn't matter). Just remember what took place and describe those things and verbalize how they made you feel... even if it's how you felt while, say, blowing him.
I'll stop at giving actual examples.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion