As an Englishman, that symbol is "sharp" (or hash) which does two things. It makes the sex sound painful, then gives you a second smirk when you remember that it means "pound (£)" in American.
One possible cause of confusion in terms of a hash (#) being called pound (£) in the US was that (as I recall even in the mid-1980s) in many cases with ASCII/EBCDIC character set terminals and printers the same number code displayed as a # in the US but as a £ (pound) in the UK (eg ISO646 vs BS4730, or EBCDIC 297 vs 37).
So when cross-Atlantic conversations occurred the US would hear/read us in the UK refer to (they thought) # by the name pound, when we thought we were describing the actual pound (£) character that we were seeing on our end.
I guess it would prevent that kind of misunderstanding if the joke was written "C:££", but then again, the slight misreading I think adds to the joke when it is gotten.
