in fact, there is no "bad sex" or "good sex". it's because we call it bad or good. It's depend on our definition of "good" and "bad".
I read an article before, it said that even you are right, while others are wrong, you become the wrong one. It depends on the values or prejudices of most people.
For example, most people are not homosexual, so we think it is bad or it is wrong. But what if one day, most people become homosexual,I think people will consider heterosexuality is bad because most people are not.