If you read through liberal commentary on the issue of sex distinctions you find certain recurring themes, all of them flowing logically and predictably from the underlying premises of liberalism:
Since liberals want to be self-determined, and sex distinctions are predetermined, liberals will often describe masculinity and femininity as restrictions on the individual. They use terms like fetter, prison and straitjacket to describe masculinity and femininity.
Liberals assume that we are made human by our capacity to self-determine. Therefore, there are liberals who believe that by dropping the idea of being men or women we finally get to be human beings.
Liberals want to be self-determined, so they particularly resent the predetermined biological fact of being male or female. Liberals are especially resentful of the link between being female and motherhood as this is held to be an unchosen biological destiny. (This is why feminists and liberals think that forcing a woman to carry her baby to term is "oppression", they didn't CHOOSE to get pregnant, they CHOSE to have entertainment, err sex, and since pregnancy isn't self-determined liberals feel they should have the power to determine what happens to their bodies, natural reproductive biology be damned.)
Liberals want to be able to transcend being male or female. Therefore, liberals often describe masculinity and femininity as being artificial social constructs, as this means they are categories that can be deconstructed. Liberals usually reject the idea that masculinity and femininity are natural, or that there are masculine and feminine essences or ideals.
Liberals want to be able to self-define. If there is only a binary choice between being male or female, the opportunity to self-define is limited. Therefore, liberals reject the idea of a binary, in favour of the idea that there are multiple and fluid sex identities.
If the aim is to self-determine, and sex distinctions are predetermined, then abolishing differences between men and women will be thought of as a liberation from outmoded prejudices and injustices. Liberals therefore believe there is a moral purpose in abolishing masculinity and femininity; it is looked on by some as a path to salvation.