It's not so much that it's a "profoundly important" event but rather that doing so can have an impact on the working relationship - and the context here is work/projects together, so that's relevant.
I work and have worked with mostly guys throughout my career. I have been asked out by guys I've worked with maybe a dozen time over the course of my career, while I was working with them. In 4-5 cases I've been asked out by guys I had worked with, but after I stopped working with them.
In the case of being asked out by someone I was working with AT THE TIME they asked, yes, it had an impact because I was put into the position of rejecting an advance from someone that I had to continue working with.
When I was early in my career, the men who asked me out were younger and as a consequence handled rejection less well than they might have if they were older - when I say "less well" I mean gossiping about me, trash talking me to other people, and in one case, trying to get me fired because he couldn't handle being in the same room with me.
Later in my career, when I was asked out, even if the men wouldn't have reacted badly to rejection, I tended to take it as significant because of my previous experiences when I had said no. At best it merely stressed me out - was this going to be a guy who acted out when rejected? At worst it got me to seriously consider leaving the company because the person who asked me out was wildly inappropriate (senior to me, worked directly with me) and I felt that my long-term viability at the organization would be threatened by saying no (and I still said no.)
The men who asked me out when I no longer worked with them? It wasn't a profoundly important event at all when they did. In a couple of cases I said yes, but in no way did I feel like "oh shit, my job is now in danger" if they reacted poorly to a rejection.
Also, on the notion of "moving slowly" - again, the context here is work/projects. This isn't a bar where people are going to be social; getting to know someone before asking them out makes sense in a work place. In fact, I would say that "someone not understanding that a work place is different than a bar" is probably one of the top reasons to reject someone out of hand.