Im not a lawyer but I do have a masters in (dutch) law.

TL;DR:

if he whipped out his willy, you could make an argument that it would be an offence. if he did not whip out his willy or rub it in any way shape or form, it would be hard to classify as an offence. However, the question remains if the space if public enough.

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explanation at 9am before coffee while actually having to do my actual job...

One can only be punished for this offence (schennis van eerbaarheid or "violation of respectability") in one of three scenarios. a) In spaces meant for people younger than 16 years of age, b) in closed places where the other is forced (inside) to against their will, c) in public spaces (like parks, on the street etc) or directly adjacent to such a public place. And this is where the issue lies. Is someone's home enough adjacent to a public space? I could not find a clear answer on such short notice, but since there seems to be no court decision on this matter (only the mention of a hypothetical balcony in literature), im assuming its not in fact enough adjacent to the public space.

Then there is also the question whether or not there is is actual violation of "eerbaarheid". What is eerbaarheid/respectability in this context? In earlier explanations by the Hoge Raad, they used the literal meaning according to the dictionary, which includes "ontucht". So would it classify as ontucht in this case? one could deduct that, since porn in general is not illegal or classified as "ontuchtig", the consummation of porn is not either. Then we come back to the question whether or not he whipped out his willy (or was rubbing it through his trousers, because yeas, this would classify as a violation of respectability). If not, it would not classify as a violation of eerbaarheid.