Yeah, and I'll agree that under most circumstances, that would be wrong. But it seems to me that it's not unreasonable to infer that the emotional trauma caused by a forced mind meld would only equal rape if the meld was not specifically targeted for certain portions of the mind and not others.
I don't think you're getting it. It's the act of invasion itself that's a violation. Which parts of the mind get rummaged into isn't relevant any more than, if you'll pardon my bluntness, which orifice gets penetrated alters the definition of rape. It's not the mechanics or the specifics of the act that matter, it's the fact that it happens at all.
Not to mention the fact that invading someone's mind by force could theoretically cause them permanent neurological damage. It goes beyond emotional trauma to something that could endanger the very self. It's entirely logical to postulate that a society of telepaths would therefore have taboos against forced mental invasion that are taken as seriously as our society's taboo on rape. That's what's really implausible about the Spock-Valeris situation: that Spock would force himself on her in any circumstances. By Vulcan standards, it should be a heinous crime.
And even aside from Vulcan standards, even aside from rape analogies, what we were shown onscreen was essentially an act of torture, and that was completely out of character for Spock, Kirk, and Starfleet. That scene was just wrong, however you define it.