Well, Godmother (and more so Godfather) mean someone of high importance in the Mafia ranks. I think he was just stating that from that moment on she would have been highly valued and, to an extent, untouchable, just my guess.Well i guess she could always be Godmother to his children.
Although I believe he plans on having her do much more than just blessing his children in the future, I reckon he pictures himself as her Master, rather than her as his Mistress. And she mentioned she likes being dominated by real men (at least that is what "my" Sophia told him)...