[Kate*]

The reason minors can't consent to sexual activity is because the law (backed by scientific evidence) has said that they are not mature enough to make the decision to have a sexual relationship because they don't understand the full extent of what that means or the ramifications of it. An adult (thelaw says 18+), has the capacity to understand these concepts and can therefore make a fully informed decision. There's nothing illegal with an adult dating a minor (say ages 18 and 16), but they can date without having sex because the minor cannot consent and may not realize what he/she is getting into even if they think they do.