The fear is not groundless. I know because it happens to me a couple of times a year. I always congratulate whoever on trying to do the best he can to make sure he leads a long and healthy life. Conversely, I have gone home with many a negative boy who found my offering of the information considerate and touching. And whatever their insistence that they would have had safe sex anyway, I am more comfortable letting them define what constitutes a gray area, and whether they are willing to go there.
Many positive guys do indeed react defensively, as if the question constitutes some personal attack.
Many would take a pass from a partner based on that information as painfully as being rejected on the basis of their looks or personality. I personally find this irrational, but then again, this is not a rational disease. As the epidemic goes on, there is an added layer of shame. Most of those now infected were armed with the necessary information on how to avoid it when they were infected. It's very human, and an understandable source of guilt.