Ok. People in relationships compromise on their preferences all the time. They do things to make their partner happy, which they wouldn't do if their partner didn't want to do it. Why is sex an area where any suggestion of compromise and having more sex than one would otherwise prefer is considered treating the less amorous partner as a "a vending machine"?

This is a way in which people compromise in relationships all the time. Plenty of couples have sex more than one partner wants, because the other partner pressures them into it. There's a big difference between this and a situation where one partner, knowing that the other partner wants it, still says no, and the other partner forces sex anyway. But that being said, couples that need to compromise a lot on things that are important to them tend to be considerably less happy together than ones who agree on the matters that are important to them, and a high degree of sexual compromise isn't a healthy sign for a relationship.