You are missing a simple reason why it is unethical and that is the general ethical principle that you should obey the law unless that law violates an important ethical principle. I can't imagine any ethical principle, never mind an important one, that would justify you in having sex with a minor. The reason why there is a (somewhat arbitrary) age specified is precisely because judgements about whether someone is mature enough to consent are very difficult. The specified age at least provides a clear test which will exclude most of the dubious cases.

@LudwigV I decided to reformulate my question; note that I am searching for an answer that lies outside of the law, as I don't think I agree with the "general ethical principle" you describe.

If Laws are in place to ensure that society retains structure and you disobey the law without good reason, you are undermining the structure of society without good reason. Is that an ethical thing to do?

There can be ethical reasons for breaking a law - especially the reason that the law itself is unethical. In that case, you would be arguing that the law is invalid. Fair enough. But I can't see an argument on those lines working in relation to this law.

I'll be interested to see your reformulation.

An interesting variation. Do you mean "able to have sex" - in which case, most people would think that puberty was sufficient - or "able to give proper consent to having sex"? I imagine you know that the age limit is proposed in order to provide an objective criterion that will exclude most of the unacceptable cases.

I understand your viewpoint, but I don't see it that way. I do not believe I have an ethical obligation to obey the law; I am struggling to put into words a precise explanation of why

"Able" meaning both physically and mentally. There are some extremely mature (both physically and mentally) underage people, and extremely immature adults. I would naturally feel more comfortable having sex with someone I felt was mature, and feel it is unethical to have sex with someone who I think is not mature. My question is how do I determine very specifically if someone is actually mature?

There would be a psychological examination in suspected cases of abuse. For example, I read of a case where a man's wife had advancing dementia. At what point could she no longer give consent?

Your question is "how do I determine very specifically if someone is actually mature?" and @ScottRowe asks a similar question in another kind of case. This isn't a concept that has specific criteria of the kind that could be the basis of a formal definition. It's a question of how they approach decisions, what they think is important and so on. Inappropriate and impulsive behaviour tells against maturity. There's a kind of circularity, because the best evidence for a decision made in one case can be the decisions made in other cases.

Evaluating the maturity of people, and their ability to give consent, is a question of psychology, not philosophy.

Maturity is relative not absolute. While it would be perfectly fine for two consenting people over the age of consent in the same peer group to have sex, it would not be if one person were 'a generation' more mature.

For over-18s, it tends to be more a question of differences in maturity and power imbalances. It's generally more moral for an 18-year-old to have sex with another 18-year-old, than it is for a 50-year-old to have sex with an 18-year-old. But it may be hard to draw any clear lines there (whether through the law or not).

@LudwigV “You are missing a simple reason why it is unethical and that is the general ethical principle that you should obey the law unless that law violates an important ethical principle.” If you hold that as an ethical truth, you should present your argumentation as to why. It could make an interesting answer. “I can't imagine any ethical principle, never mind an important one, that would justify you in having sex with a minor.” One can generate a counterexample by thinking of a bad state of affairs that could be averted by sex with minor, like the trolley problem.

en.m.wikipedia.org/wiki/Trolley_problem A really easy way to problematize the question is that different countries have different ages of consent. Assuming we do not hold that all of those ages of consent are the morally correct one, we must turn to a different argument to determine when a person is ethically sanctioned in having sexual relations.

Here is just one ethical idea that might help us in kicking off determining why anything is or is not ethical, before being applied to the specific case of sexual relationships. Is-Ought

@JuliusH. There's no possibility of replying in detail to everything you've said. If you really want to pursue this, it would have to be in a chat room. Age of consent varies, but that is not a serious problem. Maturity is not precise and different opinions are equally valid.

@JuliusH. A thumb-nail of the argument might be - Social anarchy is bad for most people. If everybody breaks the law, there will be anarchy, so that's a bad thing (in most circumstances). If only a few people break the law, and anarchy does not become general, they are free riders and that's a bad thing. That's not to say that the law is always correct; different rules may well apply in that situation.

This is one of those questions which easily deserve a book-length answer.