I can think of a few occasions over the years when I was asked this by my boss when I wasn't drinking or didn't want a pint after work - it's said in a jesting way but there's also a serious note to it. I have to say it doesn't bother me in the slightest. In fact, I tend to wind him up and pretend that I am. If the guy had come to you in work and said 'are you pregnant' then I would be a bit more upset, but if it's in a social setting and was just idle banter then I wouldn't worry about it and I certainly wouldn't make a big deal about it. It's all just a bit too PC for me - just my opinion