If it’s not too crude to ask, do we have an idea of what the immoral element of the practice would have been? I would research this myself but I can’t find the right set of keywords (and am proceeding cautiously…)
It seems there must be some additional element or motivation to have made it sinful before the Levitical law was given. If forbidden only to set the Israelites apart, then it would be ceremonial, like circumcision, which also has a hygienic benefit but is not necessary for Christians to practice.