To clarify my opinion, the distinction is a matter of viewing a relationship as a business or as something else. A prostitute is the owner of a small business, and a business necessarily transacts a clear exchange of money for goods or services with multiple customers. A marriage, relationship, or one-night stand is not a business, it's an agreement between two people. If you give your friend a beer for helping you move into a new apartment, can you call yourself a bartender? Granted, most people might call a woman a whore if she was sleeping with a guy because he buys her lots of stuff. But if she's truly a prostitute in the sense of a business, you should be able to go up to her with a wad of cash and negotiate sex with her while her sugar daddy is still standing around.
Some self-employed businesses, such as construction companies, do draw up long term contracts with a single customer at a time. However, these contracts are fixed term, with the intention of negotiating contracts with other customers in the future. Marriage contracts are, in spirit, intended to be permanent, with divorce or annulment seen as an unexpected outcome due to failure of one or both parties to fulfill the contract. To enter such a contract the way one enters a business contract is entirely cynical and counter to the purpose of marriage. Likewise, a business that enters a contract of indefinite length with a single customer (until either the business or its customer cease to exist), and is forbidden from entertaining other customers during that time, would be considered by any government to be not a business at all, but a money laundering or tax evasion scheme.
Regarding informal relationships, they are indeed often entered with the expectation of being short term. But in this case their informal nature separates them from being possibly considered any kind of business. After an abrupt breakup, nobody goes to their ex with a claim that unpaid sex or unpaid gifts are due, to be settled in court. Or at least, those who do that would be considered psychopaths by the rest of us.