For the most part, this is philosophical musings dressed up in a story. Its fun and lighthearted, and there is very little plot. When there is, it feels out of place! But as this book is meant as more of an exploration of ideas, that is what I will instead focus on.
This is an interesting piece of lesbian history, about women's rights, marriage, and social norms. Many things debated between different characters are completely normal in current society, which is a very nice reminder of how far we've come.
I do wish there was more explanation on this so called "Third Sex." It seems to just mean women who don't wish to be involved with men any more than perhaps casual friends. In short, lesbians. Not very analogous to our current understanding of gender, but interesting nonetheless. There has been a long history of women being defined as serving men, and having that heavily de-emphasized nowadays has caused gender perceptions to shift.
Something I didn't like in this story is that there seems to be an implicit biphobia in this concept of the Third Sex. You either are or aren't, and the reaction to a member of a friend group of lesbians marrying a man is a feeling of betrayal, of being lied to. Claims that she was mistaken of being a member of the third sex at all. This is the issue of drawing hard lines in the sand of any concept of sexuality and gender. They are nothing but our best shot at explaining the internal reality that is ever shifting and beyond words.