I don't think it should be considered "x-rated"... Maybe R rated, and that is pushing it. I think if a child saw this card, and did not understand the sexual innuendo, then they would likely interpret it in a literal sense - blowing out the candles on a birthday cake.
They cannot please everyone and they do have a wide audience that shops there. I think it is a good decision to cater to all than censor their greeting cards. I personally think the mentioned card would be funny to give to some certain people...