Two consenting adults can do as they please - that's the line because that's the law. When you get into the 'power dynamics' question you open a subjective shit show where any person can either pro-actively or retrospectively call foul. The only question should be.... are they both adults and are they both consenting? If the answer to that question is yes - everything else is moot.

Why? What if she's an 18 year old white woman from the US and he's an impoverished 42 year old waiter for Uzbekistan and they decide to have sex. Where's the power? Is she using her white privilege and economic status? Or is he using his maturity and age?

If you think teenagers are too young to consent, and there are very vaild reasons why that might be the case, then lobby for the age of consent to be raised to 21 or 25 when the final stage of brain development happens and move the legal framework as a protective measure. What a society can't do is begin a phased series of witch-hunts based on individual interpretations and subjective interpretations of consent, age, and power dynamics.