adorable! i loved the development of their affair, and how snape can have dirty (deliciously filthy) sex with him, but not allow sirius to give him a kiss.
you broke my heart when sirius had to find out that snape was asked by dumbledore to keep an eye on him. and i loved that showdown in the end, where snape gives up his cover to save sirius, and sirius turns into an animagus to save snape!