Hm!
Well the cornerstone is consent, but I think that there should be a bit more to it.
Agree with Marcus, that the age of majority is arbitrary and silly.
BUT. I do believe that sexual interest in the pre-pubescent, on the part of the post-pubescent, is wrong. That, to me, is a big biological cutoff point, where I do feel that the age difference of participants is meaningful, simply because of the amount of development and maturation that goes on in those phases of human growth.
I'll never, ever be ok with anyone who is, say, 13-14 and older, messing around with a 6 year old or an 8 year old or a 10 year old. Would I freak out to find that a 9 year old child was "playing doctor" (exploring sexual play) with another child of a similar age? So long as there was no hurt, harm, or coercion, I would not be appalled by that. I was certainly enjoying my own body from a VERY young age, to the point that upon recently hearing boys say that they discovered masturbation around puberty, was very surprising to me. You mean you weren't doing it well before then? I was.
I don't view humans of any age as completely non-sexual or having no aspect of sexuality to their nature, nor do I feel that, as a natural thing, it carries any inherent evil to it.
One consideration that I put out there in cases of age differences, power/authority roles, and incest... So first of all, inbreeding risk should be mentioned. That's bad. Setting that aside in the day and age of reproductive choice, and assuming that there was no coercion... Let us look at a case such as in the Jeremy Irons film, "Lolita," which my Zen recently had me watch. He felt it was a cautionary tale of how a man's life was destroyed by giving in to the sweet temptation of a teenage girl. Really now. I have a hard time seeing things the same way, because I was not merely sexually active as a teenager, age 14 onward, but I was very aggressive and sometimes pursued older men when it pleased me. What was wrong about Lolita? Well first of all I had tremendous sympathy for the mother in the story, shrew though she was. She gave her heart to this man and he deceived her, and I hate that. Deception flies in the face of consent. Then you come to the power/authority issue. It isn't merely that he is in this role...because she has little other choice in the matter, due to the situation and her place in it, her consent is thereby compromised. She cannot withdraw it effectively at any time of her choosing. She has this ongoing relationship with this man as her legal guardian, which means she can't just get away from him easily.
I don't think it was wrong for his character and hers to have had sex, if they had come together as strangers and parted ways thereafter. But for him to engineer this ownership of her, this keeping of her, I found wrong. She was not old enough to consent to serious commitment and blending the roles of parent and lover to a developing child was problematic.
I would argue the same for people in workplaces. If ceasing the relationship could mean someone has to lose their livelihood, then that is not a good relationship to be having. Same for students and teachers...although it does bring up the trickier ground of what about students and students? You cannot simply escape someone if they sit behind you in class.
Another important biological cutoff happens around age 25. This is the approximate point where one's frontal lobes are fully developed, which is the part of the brain that handles evaluation of consequences. That in my opinion would be a far more appropriate age of consent...but it would preclude a number of nice, young, healthy, fertile breeding years for females in particular. Socially speaking, it simply wouldn't fly...and with men of all ages being drawn to youth in women and having a hard enough time keeping their hands off teenagers younger than 18, I mean look at the prevalence of "barely legal" porn...for most men, a woman of 25 is nearing the end of her most attractive phase of life anyways. Sad, but perhaps true. Not for all men, thankfully...but there it is. It's a thing.
Back to the point. So when I was 14-18, I had many partners, and some few were adult men in their 20's. My first at age 14, was 19 at the time. Fortunately, my mother had the sense to get me on birth control pills, and have me visit the Health Department office for STI screenings frequently. These precautions in place, I feel that I was competent to make sexual choices and give sexual consent. What I was NOT ready to consent to, were such matters as becoming pregnant and a Mother. In fact I didn't want to do so at all, but when at 18 I was off the pill and we were using condoms, and I became pregnant, the hormonal effects caused me to fall in love with the entire business and carry the baby and keep the baby and raise the baby...just as nature intended. And because of that, I was stuck in an abusive relationship with a man 11.5 years older than I, as his third miserable marriage, and my ability to withdraw consent to it was heavily influenced by the fact that as a very young woman, I had no idea how to live as an adult, let alone support small kids, by myself. And he treated me sometimes like a child, grilling me on who I shared my time with, telling me that other men and boys only wanted one thing, demanding that I be accountable to him with no reciprocation, since he was a man and beyond all reproach.
The power imbalance there was not something I signed on for.
And it was created mostly from the fact that doing right for my offspring was more important to me than it was to him, and I knew it. But I will always feel that he came along when I was not fully mature, and hijacked my life.
At 18, I was not ready for the choices I had to make in that relationship. But hey, I made him feel good, so it wasn't unethical for him to pursue me as a means to his own happiness, I was legal and all, so no big deal.
I don't object to the sex, I object to a man coming into my life and seizing me as his property, and cementing the deal by getting me pregnant. No rules were technically broken, except that he was cheating on his second wife with me at the beginning, but still. It feels unethical. And part of that too, was deception. I told him many times from the beginning that I did not want to have kids, and he did not tell me that it was a huge life goal for him and his second wife had miscarried 18 times trying to give him a child. I didn't find that out until we were getting divorced.
I feel that had I had sex, only sex, well protected with no expectation of a serious relationship, with a 60 year old man when I was 15 years old, it would be less ethically bad, than a man expecting that at age 18 I was ready to commit to him for LIFE.
Now. The other area that, clearly, I feel is unethical, is "cheating." But again, it goes back to where I think that if deception is involved, then informed consent is not clearly possible. I also take issue with a very common practice of trying to obtain sexual consent from inebriated persons. "Picking up women at bars" is something I find deeply distasteful. I like sex. I've never needed to be intoxicated even slightly, to feel ok with consenting to it when I wanted it.