It's not about whether they'll be "professional enough" or be able to "work with the colleagues that they're given." This isn't about racism or sexism as in "can this person talk to this other person civilly?" You're working 40+ hours a week in the same office for years, and maybe the next office and the next office and the next office is the same way, and then you wake up one day and you realize you haven't worked with another member of your own race/culture/gender for 40 years and that's pretty fucking weird.
That's the track I'm on right now, and it sucks. I'm a woman who has been working in offices of almost exclusively men for over 8 years now. Once in a while there's another woman in the office in some capacity, but I haven't worked in an office with another woman in over three years, and have never worked with one as a colleague.
After work I go home to my husband, who is also a man. Most of my friends are former co-workers, all men. I spent four years in a half-time master's program and all my project partners were men. I didn't have much of a social life during that time between school and work, so I lost touch with the few female college friends I did have. I can go weeks, sometimes months, without having a conversation with someone of my own gender (my family live on the other side of the country).
The first year, fine, no problem. "Haha, yeah, token female, that's me!" The second year, starting to get a little odd. The third year, I get weird urges to stop strange women on the sidewalk and tell them lady things. I mean anything. I have no idea what women are even like anymore!
Am I professional enough to work with "the colleagues that I'm given"? Absolutely! I love my work, my co-workers are awesome, I have their back, they have mine. I think it's pretty safe to say that I do great work.
Would I be happier if I didn't have to go through my career as "the woman"? Yes. I see the pronoun "she" in the office chat and my brain registers it as my name. It's weird. I fell in love with my field but I did NOT sign up to be the asterisk of the group for the rest of my working life. I've fielded awkward questions in job interviews, weird situations from visiting executives who didn't know me, and "well-meaning" comments at conferences.
It's not about hostility or sexism, it's not wanting to have this nagging "outsider" feeling in the back of your head for the rest of your working life. It was a huge factor for me in my last job search, and I really did try to find a bigger/more diverse company, but (play me the world's smallest violin) I turned down an offer with two other women in the office for "boatloads of money and my own de facto personal bathroom." But it was a factor, and one that I'm glad people are recognizing as important.
It's not that I'm not professional enough, it's just years of sort of a weird gap in my life. Hell, if I ever did work alongside another woman I'd probably be less professional with her to start with as I unleashed years of pent-up conversation about periods and babies...