Great question, @tinyfaery, and nicely worded. I think the problem is that, obviously, there is no one magical cut-off point.
I think a situation that varies by person needs to….vary by person. Different people are going to have different capacities to be responsible and make good decisions at different ages (or at any age…), so it seems inappropriate to me to have a blanket, arbitrary cut-off that applies to all people equally.
That said, I think that if we do insist on having an arbitrary cut-off for practical purposes, the punishment needs to fit the crime and all the circumstances need to be taken into account. Sure, it might be fair to use the term “illegal” for a hypothetical 18-year-old to have sex with a hypothetical 15-year-old, but the justice system needs to be able to examine each individual case dispassionately and consider all the facts.
There is a WORLD of difference between two young people who are in a relationship and love each other very much and have sex, and an 18-year-old who finds an insecure, vulnerable girl and manipulates her insecurities to coerce her into having sex with him, and there are a thousand shades of grey in between. Go ahead and call the first situation “illegal” if you want, but does he actually deserve to be labeled a rapist? Doesn’t that discredit and undervalue situations in which someone was actually abused?
I wish @Harp would show up and solve this one…