If you've never heard an accent before (or rarely hear it) then you're bound to think that it's really strange, exotic and sexy. I think a lot of Americans will find the British accents sexy because they rarely hear them (unless they watch British television) and it's a whole new experience for them.
I live in England and I find the American accents sexy because, although I hear them quite a lot because every film I see is American, there's just something smooth and attractive about them. Irish accents (whether it's the Dublin or Belfast accent) will seem sexy to people who've never heard it before, I guess. The Irish are wonderful though!