People who are interested in extreme chassidic groups always concentrate on the members of the Edah HaCharedit without realizing that one of the most secret groups is Chassidut Gur and not necessarily the Toldot Aharon. The truth is that no Gerrer Chassid will ever publish the group's Takanot. Women are not even allowed to discuss their marriage details with their friends.
Israel's left - wing newspaper HAARETZ published an article on the gender separation in Gur:
One has to understand that Gur is not only afraid of immodesty between men and women but of potential homosexual reationships between its Chassidim. This is the reason why already teenage Chassidim have to follow strict rules in order to prevent any temptation.
One of the former Rebbes, the Beit Israel, introduced a certain Takana husband and wife have to follow. I don't have any proof for this claim but the result of this particular Takana was that Gur founded its own Beit Yaakov. In order to prepare the girls for keeping this one sexual Takana.
My personal claim is that young Gerrer women may even be unaware of this Takana, as it actually only concerns men. As a woman you don't know that something may be missing while having sex. The male Chassidim have to keep this rule and not the women.
Other chassidic groups do not have such a rule and due to that rule, no women from other groups would agree marrying a Gerrer Chassid. However, it is Gur's business what they are doing and, as a person from the outside, I shouldn't judge. On the other hand, Gerrer Chassidim are humans and humans tend to break rules. If there is a Gerrer Chassid who finds the right wife, they could break the sex Takana and keep quiet. If the Rebbe doesn't know - so, who would mind ?