I think I am making things seem worse than they are! Atheists this aggressive are very much in a minority even in Britain - I notice them more because I find their comments alarming, and they are probably also more numerous among smart-Alec Oxbridge types. This particular colleague is actually a perfectly nice young man (when you are not talking about religion) but he has a worrying over-confidence in the rightness of his own opinions, and a tendency to overstate things, and make absurd comparisons. So he is perhaps not a very good example! And there certainly seem to be aggressive atheists in the US too (judging by the books they write). But they are certainly a concern!