First of all, we must note that there is potentially a difference between a sexual behavior that is “atypical” and one that is “abnormal.” If we say that a behavior is atypical, it is just a descriptive statement. This is a behavior that most people do not engage in. However, if we say that a behavior is “abnormal” we are making a value judgment. We are saying that it is in some way morally bad. This means that there is a difference in how we decide these things. We can decide if something is atypical based on statistics, but we can only decide that it is abnormal based on our values.
Typically, decisions about whether to regulate a behavior by law are driven by two things. First, people typically want to regulate something if they think that it is an immoral behavior. This is, for example, why we make laws against adults having sex with people who are underage. Second, people typically want to regulate things if they and people like them simply do not do those things. For example, we no longer criminalize adultery (we may have laws against it, but they are not enforced) even though we think it is immoral. We do not do so because so many people engage in adultery that many people know that they or people like them do that. By contrast, fewer adults engage in sexual activity with people under the age of consent and therefore people are still willing to ban that type of conduct by law.