I'm not sure what you mean? Why would the women be inherently attractive as lab partners?
Many men in the lab would probably think that the women would be weaker, or wouldn't know how to interact with them, so wouldn't want them as lab partners.
And the men may have a higher chance of knowing each other already from outside the class in sports teams and fraternities, so they would team up with people they already know rather than someone they didn't.
But lots of men definitely do think women are likely to be weaker in computer science. I don't know why they think that - you'd have to ask them?
Yes I can. It's completely reasonable to be observe something even if you can't offer an explanation.
I don't know why some people are racist but I can see that they are.
I don't know why some people are sexist but I can see that they are.
I don't know why some people think women are inherently weaker at computer science, but I can see that they do think that by what they say and do.
If you're presenting that they don't, where is that information coming from?
I don't know how to break this to you... but a lot of people in this world unfortunately discriminate against people based on gender, either consciously or unconsciously.
> where is that information coming from
To give you a concrete example, I was at a conference just recently where the speaker took questions from the audience and when a man asked a question the answer was highly technical, and when a woman asked a similar question the answer was patronising and non-technical as if he thought the woman wouldn't understand like the men did. It was really obvious, and there was no other variable. I see this kind of thing all the time.
I don't know what more to tell you - if you can't see any evidence of men making assumptions that a women isn't going to be as competent in computer science just based on her gender then I think you must be blind to what is happening around you.