1) You did not see nuns. Formal lineage of nuns died in Theravada lineage hundreds of years ago. Women were wearing white robes right? Those are the robes of novices. You need 5? female nuns to ordain a new nun. Sri Lankan monk, Bhante Henepola Gunaratana (aka Bhante G) asked Tibetan nuns so bootstrap the tradition in Theravada, but it's just starting and there is resistance.
2) Women are considered less than men in Asian cultures (equality of sexes is new in the West too). Religions are not separate from the culture around them.
>But I am also sure, that there are Buddhist traditions and communities, which are sexist.
Yes there are and that is to be expected. (Unless you believe that Buddhism makes people somehow perfect. "After the Ecstasy, the Laundry: How the Heart Grows Wise on the Spiritual Path" by Jack Kornfield is a good book that explains how full of shit Buddhists are no matter how much they train.
Buddhism is not about creating perfect world in this world or in afterlife.