For example when he criticized Megyn Kelly as having "blood coming out of her wherever" implying that she asked him a tough question because she was having her period.
The Hillary narrative would have you believe that because he is a man that he is inherently sexist. Only a woman could represent women effectively. And of course forget that Trump had a woman running his campaign and has many high-paid women helping to run his businesses. Many people find the truth inconvenient, hence the downvotes.
http://www.cnn.com/2015/08/08/politics/donald-trump-cnn-megy...
And here is a long list of examples compiled by the Telegraph: http://www.telegraph.co.uk/women/politics/donald-trump-sexis...