All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is not permissible for a man to wear the clothes that are peculiar to women and it is not permissible for a woman to wear the clothes that are peculiar to men as this is a forbidden imitation. Indeed, we have already clarified this in Fatwa 3982, so please refer to it.
This ruling applies whether or not the intention of the spouse is to beautify himself/herself for the other.
In addition to this, such a conduct may be a reason for sexual perversion in a way that the woman would be attracted to other women and the man would be attracted to other men. Verily, blocking the means to evil is an Islamic obligation.
On the other hand, if a woman wears trousers that are not peculiar to men, then this is permissible; this is what we clarified in details in the above-referred Fatwa. This means that if she goes out while wearing such trousers, she should wear something above them which will cover them. But if she is in her home, it is permissible for her to wear them in order to beautify herself for her husband.
Our statement "provided that she does not do so by way of imitating [men or disbelieving women]" implies that if there is imitation, it is not permissible for her to wear it even before her husband.
Allaah Knows best.