We call some sex as ¡§bad sex¡¨ because it is against moral values in the society. But in addition to this reason, I think some bad sex is considered bad because there is no reason to do it or it is against what human beings like to do. For example, some people may change their sexual organs to become the opposite sex by surgical operation. This is bad because male and female are born with the ¡§determined¡¨ sexual organs. In other words, male are born to have phallus while female have breasts or other female sexual organs. Since we are born to have it, why do we need to be against nature and change it? In this point, I think we don¡¦t do it not because it is bad, but because we don¡¦t have to do things that are against nature.
As for ¡§good sex,¡¨ it also has something to do with social standards and what everyone does in the society. Basically, what is good is not because it is real good, but because ¡§people¡¨ define it good. Take heterosexuality for example. Actually, no matter it is heterosexuality or homosexuality, both of them are neutral. But when everyone in the society agrees with heterosexuality, homosexuality becomes weird and not accepted. It is the same with monogamy. In most of societies, monogamy is legalized, so most people in these societies consider monogamy good. But for some other societies, polygamy is reasonable, so for people in those societies, polygamy is good. Therefore, we can conclude that ¡§good sex¡¨ is defined when everyone allows or do it in the society.