I think other people addressed this already. But if you really wanna guarantee biological children, there're a wide array of concepts that you'd want to appeal to other than "sex," which is, again, incoherent. "Fertility" is probably an important premise. "Genitalia," perhaps.
It's common for people to refer to a sex/gender distinction. Hence strings like: "her gender is female and her biological sex is male." But it's hard to make sense of this in any literal way:
(1.) What does "sex" refer to? If it refers to a set of biological traits, then what is the sex of an individual who only has a proper subset of said traits? What exactly are the biological traits that separate the sexes?
(2.) How useful is the concept of "sex" for humans?
(3.) How do you distinguish between "gender" and "sex" in any clear-cut way?
There's also the problem of transmisogyny and transphobia. Calling someone biologically x whose gender is y is obviously not a great thing.
I would probably leave too, actually. It's a lil bit of an alarm bell re: the group.