I didn't have that big of a struggle (not married but live with my BF).
I wanted to get my second one when Ruby was still young so they could grow up together too.
All i did was say "If i can get another one then you can do ......" and just picked something that he has wanted or wanted to do for awhile. haha make it even.
Or just do what my mother did to my dad, get one without him knowing, bring it home, he'll get mad and then put the puppy in his arms and its instant love.
