It is difficult to say what Max Planck really thought on the topic, but
I suspect that there is nothing special here about Max Planck:
exclusion of women was common at that time. In many German and other European universities women were explicitly excluded by university regulations. Apparently a professor could make an exception for some particular student, but he definitely had to make some effort for this. So I interpret the phrase "unusual gesture by Planck" not in the sense that "he forbid other women" but as "he did not care to obtain a permission" for most women.
My opinion is based on a similar case (which happened earlier) when Weierstrass had to fight his university administration to allow Sophia Kowalewski to attend his lectures. Some details of this discussion have been preserved. Weierstrass failed in his fight with the university administration, and had to teach Kowalewski privately.
I remember one argument made by the opponents of Weierstrass: that presence of a woman in class would distract young male students from concentrating on the lecture.
EDIT. I think it would be interesting to make a list of the dates when women were
admitted for the first time as students in top universities of various countries.
I mean admitted in a normal way, not as an exception.
According to this page:
women were admitted to Prussian universities since 1908. This is somewhat different date from what is given tor "Germany" in the answer of Mauro Allegranza who cites Wikipedia, and the difference is relevant for the case we are discussing. The statement in Wikipedia about "Germany" is suspicious, because these questions were decided separately in each German land.