We could argue that the very nature of the "treatment" here lends extra credibility to the consent given by the patient. Spock should have been able to find out whether the consent was "for real" during the first few moments of the melding already, as (according to certain later references to the nature of the meld) he became privy to the innermost thoughts of the patient, there being little chance of van Gelder lying to him under duress or otherwise miscommunicating his will.
Of course, nobody aboard the ship could really verify whether Spock was being ethical or not, whether he was pressing on after finding lack of consent inside van Gelder's noggin. But in an environment with more of Spock's species present, such verification would come easily, which might be taken as establishing Spock's ethical qualifications: he couldn't expect to get away with villainy in the general case, so he probably would be ethical here as well.
Few real-world treatments would compare. Perhaps the closest thing would be a treatment where a coma patient is restored to consciousness, by potentially dubious means, and gains the power to either thank or condemn his doctors for the act?
DevilEyes, we reach. Thankfully, though, the actual word "rape" has not been promoted in dialogue in those cases; it's limited to an audience reaction and more or less obvious writer intent. Generally, our heroes and heroines keep such a stiff upper lip about the torments they are exposed to that we can't learn what sort of stigmata the various acts of violation really carry in the future society.
Timo Saloniemi