Someone who has sex with a ten year-old does irreparable harm to that person's psyche. Sex with a child can rob it of a quality sex life later as an adult. It's all about harms. Gay people having sex harms no one. Paid adult sex harms no one. Non-reproductive adult incest doesn't either. The rape of a child causes tremendous harm.
You've shown an example where an age of consent law is arguably too high. That doesn't mean we shouldn't have any age of consent laws. If we didn't, someone could legally have sex with a five year-old. We have to have a reasonable standard.
I'm not saying there should be no legal standards governing human sexuality. I'm saying “consenting adults in privacy” should be the standard.
What is so special about the socially constructed meanings of "consenting adults" in your proposal? How come out of all of that social construction about sexuality that you want to throw out of the window, you're clinging to this as the iron-fast rule?
If it's OK for an adult daughter to have sex with her father then why is it wrong for a 16 year old girl to have sex with a 30 year old man or a 15 year old boy to have sex with a 35 year old woman? Plenty of people used to get married at 15 and 16. They used to get married even younger. Why are you socially restricting these people from the world of "consenting adults" when they are, in fact, giving consent for these older people to have sex with them?
If you're throwing away social construction with regards to sex then why are you clinging to the social fiction that age determines ability to give informed consent. There very well could be some 12 year olds who want to have sex with a 25 year old family friend that they have a crush on and who have thought about this long and hard, who understand that older people often take advantage of younger people in order to have sex with them, etc and has weighed the pros and cons and decided that she wants to have sex. Your social construction that you impose on this 12 year old girls is that she is not mature enough to decide for herself that she can consent to sex. You don't know her, you're applying a made-up social construction to her. However, if you deny her sanction to have sex with her 25 year old love object, she can still have sex with 12 year old Bobby across the street.
There are pretty good reasons why society shouldn't encourage fathers and daughters, even in adulthood, and mothers and sons, from getting it on. You're happy to throw those reasons out the window, so why not do the same for children or do the same for children who can get parental permission to have sex. (I'd love to be a fly on the wall for that conversation, but anyway . . )
If the issues devolves to harms, then how do you balance these two scenarios:
1.) An adult daughter decides to get it on with her father. Afterwards she is horrified by the memories of that encounter.
1.) An adult woman is date raped by a man she was going to have sex with, but not just yet. She's angry that he took advantage of her in a non-violent way but she decides to continue seeing him.
Which situation produces the greatest harm? The one where the adult consented or the one where the adult didn't consent?
If the concern for minors is that harm will befall them, what special harm comes to a 15 year old girl who has a committed relationship with a 30 year old man that doesn't befall her from being pumped and dumped by the high school quarterback? The point here is that we can conceive of circumstances where same-age sexual encounters between minors produce more harm than minor-adult sexual encounters.
Again, I don't understand why you're clinging to the age-of-consent social construction when you're prepared to jettison every other social taboo regarding sex?