Help with dirty texting...haha?

My boyfriend wants me to be more aggressive, which isn't an issue in my last relationships I was the aggressor. But for some reason in this relationship I'm too self conscious because all the girls he's dated in the past have been a certain type... and I am definitely not that type. The other day he made of comment of the sort "she doesn't have a horny bone in her body, she's too sweet" which is just not the case. I want to dirty text, but seeing as he doesn't know this side of me I don't want it to seem like im trying to hard... advice?