In isolate, this is an absurd sentence. If I said "I don't think we should [do horrible thing to group x]", then you could easily retort with "yeah, but if I disagree with you I'm against group x?" in a "screw you" sort of way, and, well, it'd be true, wouldn't it?

I'm not saying anything either way about the GP, but I'd suggest it'd be better to tackle the point, rather than have such a knee-jerk reaction.

Well, if racism and sexism are real (they are) and person A believes that these are bad, then if person B disagrees with person A, that means person B believes racism and/or sexism are not bad.

Language like "racist/sexist" suggests that it's a binary, that each person is either racist or not racist. It's more useful to recognize that the dominant culture in the US is racist and that people benefit from/support that culture to varying degrees. Person B supports it.