Sex is an asset that has been used to influence behavior for as long as humans have existed. The same is true for deception, intelligence, etc. Your girlfriend could deny sex because she's pissed off with you, for example, or offer it up when she's happy.
When you're selling sex for money, it's a business transaction. It's similar to selling your body's ability to lift objects or dig up dirt.
Helping your friend build a fence is different from being a professional fence builder who sells that service.