Maybe "scientists who are girls" would be more befitting? I suppose that the phrasing "girl scientists" first emphasizes the fact they are girls before they fact that they are scientists (clearly the more relevant of the two traits).
Honestly, I can't believe you'd think that would be remotely appropriate in this context (or that I'd even have to have this discussion on Hacker News at all, or that ainiriand would feel a sense of "pride" as a male -- yeah, I checked his gender -- over a group of female biologists -- a science that is more than 50% female...)
"Guys" is casual. "Guys" is sometimes used for a mix-gendered group, but with a female-only group, you might use "Gals" or even "Ladies," but "Girls" is only used if you want to emphasize youth/casualness/fun (hence, the group of 50-something's calling themselves "the girls")
I've lived on the east coast, I've lived on the west coast, I've spent a great deal of time in the middle -- no one refers to a group of women in a professional context as "girls." After mid-puberty, that word starts to carry specific connotations when used as a description.
Again, it's not. It's really really not. Outside of pop song lyrics and college parties "girl" is not the opposite of "guy." And if you don't understand the connotations/subtle unspoken irony when a middle aged woman says "she's going out with The Girls" (and, yeah, my white-haired grandmother still says this -- she's less serious and more vocal about the phrase every year) you probably shouldn't be using the word at all when you're referring to anyone over the age of 12.
So, okay, great, you said you wouldn't actually use it. But why were you defending it, or... trying to come up with an explanation and using your 50 year old female friend as a rationalization? I don't get it.