Indeed, article 3.24 of the Arbeidsomstandighedenbesluit:
Toiletten en wastafels
In een bedrijf of inrichting zijn in de nabijheid van de ruimten waar de werknemers hun werkzaamheden verrichten een voldoende aantal toiletten aanwezig.
In of in de onmiddellijke nabijheid van de ruimten waarin de toiletten zich bevinden zijn voldoende wastafels.
De toiletten of het gebruik van de toiletten zijn naar seksen gescheiden.
My translation (intentionally less idiomatic than it might be in order to parallel the Dutch word order):
Toilets and lavatories
In a business or facility there are in the vicinity of the areas where the workers perform their work a sufficient number of toilets present.
In, or in the immediate vicinity of, the areas where the toilets are, there are sufficient lavatories.
The toilets or the use of the toilets are segregated by sex.
It's a very long law, so I haven't read it all, but it doesn't seem to impose a penalty on people who disregard the sex segregation of the toilets. I suspect that it imposes a responsibility on the employer to enforce the segregation, and presumably a penalty could be imposed on the employer for failing to do so. I do not know what mechanisms would be available to the employer to penalize an employee who uses the wrong toilet.
I don't know anything about the catering industry, and I cannot infer why you ask about it specifically. If you're asking about toilets provided by such businesses for their customers, however, I suspect it's likely to be governed by another law. (I further suspect that sex-segregated toilets for customers are optional in at least some circumstances, since small bars and restaurants often have only one toilet.)
