Fair enough.
In which case, the best way I can think of to explain it is this.
There are two things, sex and gender. The various competing belief systems choose to accept or reject either one or both of these things.
Group 1: rejects the reality of sex, accepts the legitimacy of gender.
Group 2: accepts the reality of sex, rejects the legitimacy of gender.
Group 3: accepts the reality of sex, accepts the legitimacy of gender.
Group 1 are those who believe that trans women are women and trans men are men. They believe that having a penis is not an impediment to being a woman and having a uterus is not an impediment to being a man, because what really makes someone a woman or a man is what gender (i.e. presentation, roles, behaviour etc.) you identify with.
Group 2 are gender critical feminists. They believe that men are adult humans of the male biological sex and women are adult humans of the female biological sex. They find gender regressive and believe that people of either sex should be allowed to present and live however they like, provided they aren't harming anyone. They do, however, believe in the importance of biological sex in certain situations, for example, sports. They believe that women should have equal opportunities to compete in sports as men do, but not that women should actually compete against men, because they recognise the reality that women and men have different bodies, making women uncompetitive against men in sport, but men unable to have babies.
Group 3 are social (and often religious) conservatives. They believe that if you have a penis you are a man and if you have a uterus you are a woman. They also believe in gender and believe that men should conform to masculine standards of presentation, roles and behaviour and that women should conform to feminine standards of presentation, roles and behaviour. They believe that sex is inextricably linked to gender, and anybody who rejects the gender roles associated with their biological sex is effectively breaking the social contract. This is why they are both anti-trans and anti-feminist. They see trans people as perverted for wanting to perform the gender roles and stereotypes typically associated with the opposite sex. And they are generally opposed to women's rights because they see women fundamentally as breeders, who should spend their fertile years barefoot and pregnant and their whole lives taking care of the family and the home.
This is important because both Group 2 and Group 3 define men and women according to biological sex, not gender. This gives those in Group 1 the ammunition to say that Group 2 are allying themselves with Group 3 (despite the fact that most of us abhor Group 3 and agree that they are not feminists and do not have women's interests at best heart), and to claim that defining women as breeders is regressive.
But it is a fact of life that we are the ones who have babies. That is the very reason why we need sex specific rights (to prevent us from being fired for being pregnant, for example) and sex specific healthcare (such as the right to an abortion). It is very very difficult to fight for the rights of a group of people you cannot even name.
So no, it is not regressive to define women as the childbearing sex, because that is what we are and it is why we have specific needs that men do not have if we are to participate equally in a world that has been built by and for the benefit of men.
Personally I think the alternative viewpoint - that women are people who identify with the gender norms typically ascribed to the female sex - is regressive. Feminists have been fighting for decades for women not to be limited by these things, and for us only to be treated differently to men where there is a physical reason justifying such difference in treatment (i.e. asking people to give up their seat for a pregnant woman on the bus). The idea that a woman is something entirely nebulous and subjective, something you can choose to identify as or not, is hugely unhelpful in that regard and risks undoing decades of work by feminists to build a better world for girls and women.