I personally do not think that what goes where in a sex scene has anything to do with who holds the power in the sex scene. Which character is the dominant character in the sex scene – if there is one – or who is in the position of power in the sex scene. To me, there is nothing inherently dominant or powerful in putting one thing inside of another thing. If you construct that act as powerful it’s because you think that the object being inserted is somehow more powerful than the thing it’s being inserted into. You’ve somehow come to imbue a penis with additional power and constructed it as more powerful than a vagina or a butt or a mouth. Think of it this way – you don’t think that you’re dominating a shark when you stick your arm in a shark’s mouth. That’s because you think of a shark as more powerful than an arm.