Can't really say that the company in the OP is inherently sexist because it has no women on the shop floor. Some manufacturing industries have a high proportion of rather old fashioned processes, such as milling and grinding; I have worked in manufacturing environments for 12 years and I have never seen a woman work on the shop floor in these places. And I have never met one who would want to.
Also, there is a very distinct divide (rightly or wrongly) between shop floor staff and white-collar staff in manufacturing environments, large or small. I work for one of the largest American companies, who really work hard to promote and implement equality, and this is still the case.
To be honest, I don't think it is unusal for a shop floor workforce to have a jolly. This has only reared it's ugly head because someone (idiotically, admittedly) decided to have a separate wives and kids day. The majority of companies would not bother with a wags day out, it would have been blokes to Cheltenham races, and that would be the end of it.
Regarding the women who work in the offices not being invited, I would be extremely surpised if the office/managerial/admin staff did not have a separate, work funded day/evening out. I have never heard of the shop floor having a jolly and the office not. There would be uproar!