What.. "Lesbian? Her birthdays in March. I thought she was a Pisces." means?

What.. "Lesbian? Her birthdays in March. I thought she was a Pisces." means?

I know it's from bend it like beckham, but I don't get this part?
I've never seen the movie
Oooh, wow. I get it now.
I guess I was reading it as if she was a lesbian for a Pisces, but I get that they thought it was a horoscope! thank you lol!