Shifty Sex — Susan Hart

Shifty Sex, is a story about a shapeshifter who hasn't told his girlfriend yet about his ability to change his form every month or so, most of the time unwillingly. When they are making love he starts to transform even though he's trying his hardest not to, and when this lover turns on the light she is, naturally, horrified. Who would want a bunch of scales and green slime to get in the way?