It's not a circular definition. "Obscene" is a term used for content that has been deemed illegal. It is not a general term for sexual content. It is not a term for content that is illegal sometimes but not other times. It is not a euphemism for material that is restricted to adult consumption, or for explicit pornography.
more charitably, but still repetitious: when one says that something is obscene, that's saying that it should be illegal in any context; that it has no value. A drawing of two people sucking each others dicks has surely met that bar in the past - information about birth control has met that bar in the past. But I do not think the suggestion was that drawings of gay men having sex should be illegal, what was being suggested was that it is not appropriate for children. That's not a question of obscenity.
more charitably, but still repetitious: when one says that something is obscene, that's saying that it should be illegal in any context; that it has no value. A drawing of two people sucking each others dicks has surely met that bar in the past - information about birth control has met that bar in the past. But I do not think the suggestion was that drawings of gay men having sex should be illegal, what was being suggested was that it is not appropriate for children. That's not a question of obscenity.