The concept of consent is meaningful here:

Person A: "Wanna fuck?"
Person B: "Sure"

^^ Is consensual

Person A: "Wanna fuck for money?"
Person B: "Sure"

^^ Is also consensual but the government has made that consent illegal.

Person A: "Wanna fuck?"
Person B: "Sure"

Person C: "I don't want those two people to fuck" <--- Person C is not relevant regarding consent in this matter.