I don't believe that the term 'offensive' can be used in this category - it was a simple error that arises from language usage over many many years. We all use generic terms at times.
I also believe the word 'offensive' has become synonymous with 'I can't handle the slightest little thing said to me that I disagree with'. It is, quite simply, an extremely overused word.
When I'm travelling, I love visiting cathedrals, churches, castles, etc. If, when I'm visiting these places, people assume that I'm Catholic, Muslim, Buddhist, whatever, it wouldn't 'offend' me at all. I may point out that I am not XYZ but the chances are I simply wouldn't bother. Reason: it wouldn't offend me but might embarrass the other person.
If people can become offended and outraged by something such as being asked 'what is your Christian name' (in a country that has long used this term in every day life of every day people) then they really need to look to their own mindset.
A simple 'Actually, I'm not a Christian and prefer [whatever]'. Dealt with simply, politely and with tact.
If the comment had been made with the intention to offend, that's quite different and that is when I would deal with it quite differently.
Then again, I'm not given to overreacting to situations so I find it an easy thing to 'deal with'.
There are many things said in our society that I find offensive and those I would deal with. Astro, so why isn't Hipscarf allowed to do the same?