It varies, but usually I try a little ass play and gauge the reaction or bring it up subtly and go from there. Some women don't dig it....and I'm cool with that. I won't enjoy it if the don't enjoy it.
I have a bud who is a carnivorous ass whore with the ladies....he is like a real life Rocco Siffredi. He is pushy and forward enough that he gets that ass on 90% of women and gets it quickly. He has his own method....the A.M.P. Method.
Ass then mouth and then pussy. He figures if you conquer the ass first, it's all easy from there. I have verified this with some of the women he has had sex with and they don't know why they did it, but they did.
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