i believe it's simply a matter of who is the most dominant in, if not the relationship, then the particular sexual act.. at least it is for me.. i normally have a bit too much pride to say that my wife "fucked me" and in practical terms she hardly ever does.. i generally "fuck her" though our case might be a bit special as we have a definite top/bottom mentality in our relationship.. not to mention our sex life..
^^
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