Why is it so unbelievable that people would say this? Some celebrities are willing to straight up say to the media that they won't vote for a female nominee:
http://time.com/4071617/ti-woman-president-hillary-clinton/In a March 2016 CNN poll when asked if the country was ready for a female president, 19% of respondents said no. This question is often a more effective way of getting a sense of the level of opposition to electing a female president than bluntly asking "would you vote for a qualified female presidential candidate?"
http://www.cnn.com/2016/03/04/politics/female-president-poll-results-cnn/index.htmlThe current numbers of 80% ready/19% not ready are considerably better than in 2008. At that time, only 67% felt the country was ready and 32% thought it was not ready for a female president.
1 in 5 voters thinking the country won't elect a woman is a minority viewpoint, but it certainly isn't super rare. It's easy to believe Clinton has encountered some of these voters.