(1) Even if I really believed -- which I don't! -- that 98.07% of British men enjoy having sex with other men, I would hope it should be rather obvious by now that I don't personally consider it insulting to suggest that a person is homosexual -- for the same reason that you wouldn't consider it insulting if I said that 98.07% of British men are secretly Swedish.
(2) In any case, it was a joke -- the point being that to make guesses about a man's religion because his name is "David Goulding" is as ridiculous as making guesses about a man's sexual orientation because he's British.
PS Of course, there ARE names -- or forms of names -- that are particularly associated with one religion or another -- a Christian boy is unlikely to be named "Shlomo", a Jewish boy is unlikely to be named "Christopher", and neither of these boys is likely to be named "Muhammad"! However, neither "David" nor "Goulding" happens to be one of those names that are strongly linked to a particular faith, at least in English.