I've always found it funny how modern society is basically formally libertarian about sex and not nearly anything else.

Are we? Western republics put a large subsidy on long-term heterosexual monogamy. While the law is difficult to enforce, marrying someone without romantic intentions is a felony in the United States.

Age of consent laws are supposed to protect children, but selective enforcement is often based on the sex of the perpetrator. (The sex of the victim doesn't matter as much. Men who sleep with boys generate about as much outrage as men who sleep with girls, while women who sleep with boys generates as much outrage as women who sleep with girls.) Sometimes the age of consent laws themselves are discriminatory. The US Supreme Court has held that because girls risk pregnancy, states can impose gender-based ages of consent even for oral and anal sex. And while we're talking about anal sex, Canada's age of consent is higher for anal sex than vaginal, unless the anal sex is between a husband and wife.

Incest laws are interesting because they contradict the progressive narrative of a widening tolerance for different kinds of consensual sex. Politicians speak of incest as if it were always non-consensual, such as when they defend abortion rights for cases of "rape and incest". But even in cases where consent is recognized, this is not always a legal defence.

Can't forget about prostitution laws.

Polyamory is legal if you don't seek the government subsidy, but local regulations often prohibit too many non-relatives from living together. Actually, any alternative lifestyle can be crushed if it offends the homeowners association.