We give conditions because we know there are potential pitfalls. To put things in perspective, ask someone how to have an adult/adult sexual relationship and ensure that the sexual aspect of the relationship doesn't lead to harm; I bet they'd give a number of conditions, likely more than those present in a child/adult sexual relationship.

And, of course, adult/adult heterosexual relationships aren't seriously taboo, which means that no condition is needed to address the problem of it being such.

Also, most of our conditions are obviously required for a healthy sexual relationship, or required to avoid violating rape laws.