So it's sexist to call someone pretty? Or it's only sexist to call someone pretty if it's a man calling a woman pretty? Or is it only sexist to call someone pretty after saying she's smart? Is it sexist to tell your wife she's pretty? Sorry, for all the questions, but this sounds suspiciously like a no-win scenario. In other words, "it's sexist, until I don't want it to be sexist."
Maybe "pretty" wasn't the best example. Just telling someone they're pretty isn't sexist, but it does presume that they care and that it is important to them. And, yeah, I chose how I phrased that.
The problem isn't that guys in the skeptical movement call women pretty or even that they (along with some female skeptics who shall go nameless <_<) objectify women occasionally (hello explicit thread). The problem is that it plays up stereotypes. I think there was a similar discussion when Kirsten Sanford was on the show. A lot of guys were all "she's pretty, we need more smart, beautiful skeptical women," etc. I'm paraphrasing of course, but why did the fact that she was attractive even need to come up? Yes, she is, but what does it have to do with her science? And it does happen occasionally the other way around, but we're talking disproportionately. And even that can be a problem. Who cares how attractive someone is - when the topic is science, let it be about science. When it's about who's hot and who's not, then we can bring out the mud baths and wet t-shirts.
I guess I'm trying to say that it does depend on the greater context of the conversation or discussion being had. If you're talking about neuroscience, and quote Dr. Novella, it is sexist to add "the dreamy" in front of his name. If you're talking about Dr Sanford and want to make a point about how smart she is, adding "pretty" to the mix just doesn't make sense. Her intelligence and beauty are mutually exclusive.
And this is a tangent, but I got to thinking about some of my experiences on this board. How many of you, based on my posts and the way in which I interacted, made the assumption that I was a dude? How many "omg panda's a chick" posts have been made? Some of you made assumptions about me based on my posting method (well, that and my "boobs rool" comments, sure) - granted, I don't post like a chick, I'll be the first to admit it. But you (general) saw how I was interacting with others on the board and added it up in your (general) head and came to the conclusion that I was male. I'm not trying to argue that this is sexist, but just that assumptions are made and sometimes they're grossly wrong. Sometimes women don't want you to tell them they're pretty, just like sometimes guys don't want to hear about how cute a woman thinks their butt is.
Oh, and footnote, there are some people that way overreact about these things. We call them "crazy".