I can't find the quote now. But basically it boils down to whether you see gender as a form of self-expression, which clearly comes from the person themselves, ie, it's innate, and also means that different activities and identities can legitimately be classed as masculine or feminine. With this viewpoint it makes sense that gender would be on a spectrum and it's still okay for people to move or flip between gender roles. I don't know what this POV is called, I used gender activists earlier but was told that was definitely wrong.
The other POV in the debate is that you see gender as a tool used to oppress, police and restrict people, ie, it's external, and possibly non-existent. Biological sex clearly exists but there is no such thing as a masculine/feminine activity other than those restricted by biology, e.g. childbirth. Cooking, engineering, fairies, gardening, music, make up are all just interests that people might have and have no relevance to their sex. This is the Radical Feminist position.
Then there is the conservative POV which is that biological sex and gender are inextricably linked and should never be broken, but I don't think we are talking about that at all (though sometimes I think the RadFem POV is mistaken for this one, which it absolutely is not).
As I understand it, the first two views are totally incompatible. You can't hold both at once. The problem with talking about the issue is that we use the same words to denote biological sex as those we use to denote gender, so you get people talking at cross purposes and thinking that the other person is being totally ridiculous when actually you just aren't respecting where the other is coming from.
It might be useful on these debates to specify whether we are talking about biological sex or gender when we mention "men" "women" "boys" or "girls". For example, the part of the RadFem argument which often appears to be anti-trans is concern that issues which disproportionately affect bio-women are becoming more difficult to talk about because of either laws which allow people to change their legally recognised gender, which in everyday life does translate to sex, or language issues, for instance stating that both men and women can have wombs and so face the issue of abortion. Well, perhaps people of a male gender and people of a female gender might both have wombs, but it's not true that bio-men have wombs, only bio-women. It's not anti anybody living any lifestyle they wish, it's anti minimising of women's issues.
I find it very difficult because I do think that there is some common ground but both sides are so very offended by the others' point of view that it's very difficult to arrive at that place where we can work together and so it's become some sort of battleground.