It's a bit like asking a woman why she's a woman. But before she answers that question, she needs to take account of the following:
Not all women have wombs, so that can't be the definition. You can be, after all, a trans woman without a womb, or a woman who has had a hysterectomy, to mention a couple.
Not all women have estrogen in their systems. It's possible to be a woman who is menopausal, for example. It is also possible to be a trans woman who does not take estrogen. So estrogen can't be part of your definition either.
Not all women give birth or are able to give birth.
Not all women want to give birth.
It is possible to be born female and have a penis. You can be intersex. You can be transgender.
Not all women have breasts. Some women are born without them. Being intersex is one example. Another: you can also be a woman born without any breast tissue. A third: you can have your breasts removed. Tig Notaro, in fact, has some interesting things to say about being a woman without breasts. Check out this piece about her topless set.
It is possible to be a woman without a vagina/vulva. Kaylee Moats, born without a vagina, vulva, or cervix, bravely speaks out about feeling "less of a woman," though in fact, she is absolutely a woman. And that's just one of many ways of being born without a vagina/vulva.
It is possible to be a woman without having vaginal sex.
It is possible to be a woman without having any sex or any relationship with anyone of any gender. It is also possible to be a gay or asexual woman. So you can't say being a woman is about having sex with men.
It is possible to be a femme man. So being feminine in appearance or energy isn't uniquely female. It's also possible to be a butch woman, of course. Take this into account when you give your definition.
It is possible to long for children and be male.
It is possible to give birth as a man. It happens all the time. Just type "trans man gives birth" into Google.
It is possible to be a woman without dressing femme. Check out, as an example, the awesome Dapper Q.
It is possible to be a woman who often reads as a man. I've met a couple of folks who were born as women, but have been thrown out of women's restrooms under a cloud of devastating and cruel suspicion on many occasions. The gender binary is brutal.
You'll notice that all these points are to do with the physical body, which is often how the world erroneously sees gender. Gender is actually a construct. And biological sex isn't binary. If you don't believe me, check out this piece at the Washington Post by Juliet Jacques.
So , considering that you need to leave out all those arguments we've crossed off the list, how do you explain what it means to be a woman? Hard, isn't it, when you can't use your body to define your gender?
Assuming you're not a bigot who denies all the above points, this is how I feel when I'm asked what it means to be non-binary. I'm not a woman, because I've never felt that fitted. I'm not a man, because I've never felt that fitted.
Truth is, I'm non-binary. I just am. Trust me.
Thanks for reading, folks.