The wonderful Yahooey would argue that it's only social conditioning which tells us that anything is sacred. The way our society works is by a communally agreed upon set of values and standards. Thus, while there may be minor differences, most people in our society agree on the same things being of value.
The wonderful Yahooey would also like to point out that more specifically, a relationship between two people relies on those individuals having similar values, or compromising on the values which are different. Thus, for the majority of people in our society, the genitals are considered private or sacred, and if he is not willing to compromise his differing values on this, then there will be problems in the relationship, in much the same way as someone with vastly different values from the rest of society (murderers, thieves, naturists, anything vastly different) will experience problems functioning in mainstream society.
The wonderful Yahooey has spoken.