french letters...

[Burgess, John]

I suspect that, much like "French pox" v. "English pox" (i.e., syphilis) 
and "French leave" v. "English leave" (i.e., running away from battle, 
slipping out the back), this is just another trans-Channel slanging 
match.

While the English do use "French letters" for condoms, the French also 
use "English letters" for the same.

This titbit, however interesting it might be, may not be exactly what 
you're looking for, i.e., "why 'letters?'"

Other than the simple observation that a condom is somewhat akin to an 
envelope, I don't have a clue... maybe that OED on CD-Rom that's knocking 
around the ListServ might help?