Employment shapes society. Employment pays taxes, puts rooves over people's heads, causes stress and worry (what if I lose my job!?), informs societal choices regarding basic social services (tied to company healthcare). Finally, employment is a public act because it is a market -- if Joe works for free, that creates market pressure for Tina to work for free, too.
With the exception of (almost entirely illegal) sex trade...
1. Sex does not house people
2. Sex does not employ people
3. Joe doing it up the butt does not affect your ability to find a partner
4. Tina engaging in fellatio with her husband does not affect your ability to find a partner
5. Couples not having babies does not affect other couples' ability to have babies
6. None of this affects healthcare or taxes
It's simple: it's not a market.
That's why employment is a public act.