I recently heard someone make an argument, something like- "if you accept that there is morality in sex, for example that a father having sex with his daughter is wrong, you can't say gay sex isn't immoral because people should be able to do whatever they want because it causes no harm to others" Is this argument or proof begging the question? Philosophically, what is wrong with this argument.

The main thing wrong with the argument is that it is terrible. Don't we think it's wrong for parents to have sex with their children precisely because we think that it is harmful to the children? One might also think that children have no genuine capacity to consent to sex, an issue that also arises in other settings, such as between a boss and an employee. In such a setting, there are always issues about coercion, even if such coercion is not explicit.

Presumably the thought is supposed to be that there are forms of sex that are morally suspect, even though they do not cause any sort of harm. But then one wants to know what those are supposed to be. Then we could consider whether and why they are morally suspect. The example given, as I said, is a very bad one.

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