Yes boners aren’t alway a sign of sexual arousal, but even if the person IS sexually aroused it doesn’t mean they want sex. A boner is not consent. a boner might mean they could be scared, tired, energized, uncomfortable or they are aroused but JUST DON’T WANT SEX.
Prime example: “God dammit I have an awkward boner in the middle of class”
*teacher voice* “Learning is exciting. You should get a boner!”