Prostitution is legal in my country as well as several other European countries but yes in countries where this isn't the case this seems an unprincipled exception. Incest is the big inconsistency nearly everywhere and I've been puzzled by this in the past.

In any case I wasn't talking so much about law but about the kind of ethical reasoning that is acceptable on the matter in polite company. The ethical ideal that people claim to aspire to is very much "between consenting adults". Though that may actually just be poetic language for the tribal attire that corresponds to "I think gay sex is fine.". But even if so libertarian ideals let alone practice are something we've been on net moving away from them on most matters in the past century at least, but moved towards them when it came to sexuality, this seems an anomaly.

With regards to marriage. You aren't under legal obligation to have sex with your married partner, so most of those points are I think covered by this:

Now obviously our society isn't really libertarian about sexuality. We seem to regulate to death with social and legal norms nearly every aspect of interhuman interaction that is related to sex but isn't sex. This contributes to the desirability of a bare bones approach to sex logistics, the one night stand, if one is doing cost benefit analysis.