i want to be clear: i agree that this is a stupid debate. it's pretty clear what the slogan means. but, i think the proper response to this is to address and react to two concerns:
1) why are you getting this response? that is, why is it unclear what the slogan means? "white people are stupid" may be true (no! all people are stupid!), but it's not good enough.
2) can you modify the slogan so that it doesn't create confusion? i mean, (1) implies this is necessary, if you're being empirical about it. "all lives matter" doesn't cut it. but, clearly, "black lives matter" isn't getting across what you're intending.
i think that what is missing is just one word - "too".
black lives matter, too.
this gets the point across clearly, without accidentally suggesting something you're not intending. it's actually more direct - it's precisely what you mean to say.
but, why do you need to do this?
see, i think the other part of this is realizing that the push back you're getting is out of a legitimate sense of economic desperation. if you look at poverty rates and unemployment levels, there's a lot of white people in some bad shape right now. this is another issue unto itself. activists will tend to cite statistics that say that white people are disproportionately ultra-wealthy (that is, the "1%" is disproportionately white), and black people are disproportionately poor - and this is certainly true in most of the united states. but, they won't cite the stats that also say that whites make up the single largest ethnic group in poverty in the united states, and that this is a phenomenon that is increasing. there is a real phenomenon in the united states (and also in canada) where the middle class is becoming increasingly ethnically diverse, and whites are increasingly occupying the bottom of the income scale (while the elite remains disproportionately white). if you look at stats in canada, whites are actually fourth in terms of average wealth - east asians are the most wealthy people in canada, followed by south asians and west asians (which means muslims) and then whites.
that's not a klan fantasy. that's the statistical reality of the situation.
but, there's this constant stream of information bombarded at whites that tell them they have "privilege", that they exist at the top of a hierarchy, etc - when any data you can gather suggests that, on average, whites are in fact a moderately to severely disadvantaged group in the united states - despite being disproportionately represented in the media, in the super rich, etc. it's easy to see how they could misinterpret a slogan like this badly - that is as suggesting that they don't matter, in an economic climate where this is increasingly reflective of how they're feeling.
and, once you get this, it's easy to see how the request to modify the slogan is actually a request for inclusiveness - that is, a request to build a broader based and more class-focused struggle. and, maybe that's a worthwhile request to consider, even if it requires a little decoding.
i'm not suggesting that a lot of the reaction isn't racist, because it is. but, even when it is, it shouldn't be completely ignored. it should be analyzed and understood.
but, i think it's also important to take responsibility for poor messaging. i don't know who came up with this. i doubt it was focus grouped, or sent through a marketing evaluation; it was probably literally some kid that just typed it out without thinking about it much. throwing it out in the wild, though, is doing that. and the results suggest that some kind of adjustment to make the messaging clearer should be embraced, rather than rejected - it's clearly entirely necessary.