Is being offended by how someone dresses ever ok?

On the 4th March 2015, the Independent newspaper commented on Professor Richard Dawkins' Newsnight interview with Evan Davis. In the interview, Professor Dawkins said he was personally offended by burqas.  This led me to think, is he really offended by burqas or is he perhaps offended by the reasoning behind why he believes people (ladies) wear burqas.

I doubt that Professor Dawkins is offended by clothes that cover the body. I suppose he isn't offended by a space suit or a diver's frog suit. Same for a balaclava when it is cold in winter. Also, would he be offended by traditional Arabic dress for men which also covers the body?  I would doubt it.

I really don't see how someone can be offended by clothes or more precisely by wearing clothes that covers the entire body. The offence must be at the reason why someone dresses the way they do. But this is a tricky area when a man comments on what he thinks a woman should wear or he is offended if she dresses in a certain way. I wonder what clothing a woman could wear which doesn't offend Professor Dawkins.

The comment from Professor Dawkins reminded me of a joke. It went as follows:

3 ladies were interviewed by a male store manager for a job as a cashier. In each interview, the ladies were asked what would they do if when they cash up their till at the end of the day there is a surplus of US$50 in the till. The first lady said she would keep the cash in her till and inform her boss the next day. The second lady said she would wait until the end of week and see what the discrepancy was before informing her boss. The third lady said that when she worked as a cashier before she would often use her own money for needed change and put it in the till so any positive discrepancy would be hers. Who got the job?

The lady wearing the tightest shirt.

This is a joke to show obvious sexism which is deplorable. But I would ask is it ok to be offended by the fact someone is covering themselves? Can you allow this to creep into your judgment when you are conducting an interview? Can you actually remove your personal offence from such a situation? Indeed, in the interests of avoiding sexism, isn't it unhealthy to hold such a view or at least unhealthy to pre-judge why people dress the way they do?