And we can't have that, because it (to their minds) creates a slippery slope where their own personal perversion can be similarly questioned.
In case you're wondering, I have two daughters and am fully in favor of death penalty for "actual" rape. The only two problems are the idea of executing someone for actions that are wrong illegal only because of the POV of the "victim," and the possibility that capital punishment for rape provides an incentive to murder.
One result is that "Natural Law" reasoning, once accessible and persuasive to anybody with a reasonable familiarity with human nature, now commonly fails, at least rhetorically, because both "reason" and "human nature" strike people as being either entirely fictitious, or impositions on their liberty.
You could get the same result by saying that the decisive factor is the point-of-view of the perpetrator. He knows whether the actions he's performing are acceptable to their object.
That aside, I think you've made several very going points. Another problem with the "consenting adult" standard is that there is little rational basis for limiting "adult" to any particular age. Our current legal standard says that a person aged 15, for example, is "incapable" of consenting to sexual activity, but then says that the same person is capable, if the other party involved falls into certain categories. And all sorts of things are excused simply on the assumption that both parties got some physical enjoyment or emotional benefit from it.
Given the current environment, it's hard to see where a defensible line can be drawn short of "too young to talk" or "violent physical coercion."