I don't think it creates a 'hostile' atmosphere per say. Obviously if both parties are alight with the situation then it's fine for them.
I think it's an inappropriate one for a workplace though. This comes down to company culture. The problem isn't the women hula hooping, a casual environment means such things will happen. People will exercise together, work on hobbies, etc.
The problem is that in a work environment everyone who works there has to be treated with respect, and equally, and that means not cat-calling your co-workers. Even if the women were fine with it (and they probably were since they didn't stop), the large group of men treating them like sexual objects in the workplace is not conducive to an environment where women are respected. If this was a strip club there wouldn't be a problem.
To draw a parallel, if 7 out of 10 people at the office were having nerf gun fights, and it was disrupting other peoples work (because they are reflexively duck, etc) to the point where they can't work when there aren’t big nerf gun fights (but still one or two shots every now and then), then it's a problem, and destructive to a good work environment. This is where management needs to step in and say, "ok guys, I know we are pretty lax around here, but seriously cut this shit out". Obviously it isn't a perfect parallel.
It's also a culture fit thing. This was the thing that broke the camels back. This employee suffered discrimination from executives (and startup founder's significant others are likely going to be part of the executive team one way or another. They certainly have the standing of the executive team in many ways. Legally speaking they are almost the same 'person' in many ways), she suffered discrimination from a single employee, and then she suffered it from a group of employees (a group mentality). And all three of these things feed into each other, and in a work environment (especially a casual one), employees need to be able to move their bodies without worrying about a group of other employees demeaning them.