What if someone wants to engage in sexual masochism, where "harm" is deemed sexually exciting?
"Harm" should be considered something someone can't heal from. If some one likes being whipped, that's fine, but if you want to stab someone or cut off someone's limb, that's not.
Define 'adult'. Is there an objective definition for "mentally developed"?
A mentally developed adult is basically able to manage their emotions and impulses. Some "adults" never seem reach this stage, but for an average person, it's basically near or sometime after puberty. We may not be able to say definitively when that is for everyone, but we can generally estimate that it is sometime after the late teen years. There has actually been research done however, that showed that the average age at which people's brains are fully "developed" is around age 25.
What about if two kids want to have sex? Is one kid pressuring the other kid? Should we separate the sexually pressuring kids, from the kids who are pressured?
Both kids may want to have sex, but they should still be discouraged from doing so, in part because of the issue of "development" I mentioned previously and also because of the reason I mentioned later for what I think should be the actual standard for beginning sexual activity, which is to be able to care for oneself completely. Of course, there are also clear physiological reasons why really young kids shouldn't be having sex as well.
So, if two animals are having sex, neither one is giving consent?
No, not in human terms. No more so than a small child can be considered to "give consent" to anything. Or course, it's fine for animals to mate with each other because they aren't held to human standards. Just like it's fine for a lion to kill and eat a zebra.
Interracial relationships once created (and still do create) conflicts in families.
Interracial relationships don't inherently create conflicts within families. That's an issue with personal perspectives and bias. And the interpersonal conflicts that those relationships might create are not the same as the damage incest does to the functional roles family serves in a person's life. Lovers may come and go, but sisters, brothers, mothers and fathers are usually a part of a person's life for the duration of their life, without the burdens of romantic expectations, which is a beneficial dynamic to maintain.
Damaged genes could be easily corrected, in the near future, via genetic engineering.
Unless you've seen some research that I haven't, that's just random speculation. But even if that were the case, there is also the issue of limiting genetic diversity. The benefit of mating with others you're not related to is that it introduces variation in the genetic code which contributes to our survival as a species.