Didn't we just open the door to breast size stereotypes though?
Do you mean
only that stereotype? No. You open the door to any stereotype that has a modicum of evidence to support it. As soon as you accept one racist stereotype on that basis, there is no logical reason not to accept them all.
Consider the most important example of a racist stereotype; that black people are not as intelligent as other races. There is even some evidence to support that argument -- black people do indeed have lower average IQs than do people of other races, both in the US and outside it. There are two reasonable, serious arguments against this racist stereotype.
1) Race is largely a sociological construct, and there is unlikely to be any real, significant genetic differences between the races outside of different melanin levels.
2) The evidence to support the notion that blacks aren't as intelligent is hardly conclusive. IQ is not a perfect indicator of absolute intelligence.
The problem is that we've already eliminated
both of these arguments as soon as we accept that penis size "myth.". If we accept the penis size "myth," then we also admit that there
really are real, significant genetic differences between the races; so just as one race may have a larger penis size than another, so too might one race be more intelligent than another. And If we accept the penis size "myth" based on inconclusive and incomplete evidence, then surely we must do the same when it comes to myths about racial intelligence; in both cases, the science is incomplete, but they sort-of-kind-of point to both myths being true.
You see the problem? The rational defenses against the more terrible stereotypes of the world are thrown out the window as soon as you start accepting less serious racial stereotypes, and do so based on scant evidence. Again, it's a door I'm willing to walk through and explore, and it's all in good fun when we're talking about something that doesn't matter all that much like penis size, but I don't think people are going to like the more serious, harmful stereotypes that walk through the door along with it.