About Margaret, I'm not all that sure she (or any other amazon) will get asked to join. But even if she is, she might refuse anyway.
As for only girls being born, I wonder if it's something like this: women in the pirate crew sometimes get involved with men (to some degree, they might have to in order to keep their numbers up). When they get pregnant and the baby is born, they could either leave it with the father or an orphanage if it's a boy and take it along if it's a girl. Or maybe they just really mysteriously only have girls. Strong amazon genes?
After reading the chapter for the umpteenth time I was just thinking that Luffy may have corrupted all those amazons. They'll all be curious to see the kintama if they ever see a man again.
Also amazing that he's more worried about being cold than being naked in front of a bunch of women. One thing I liked as well was when he was avoiding their attacks and he's bouncing around in a sitting position thinking about the vivre card. Well, can't wait to see what Boa Hancock is all about. My guess is neither Yonkou nor Shichibukai btw.