Hah! I wouldn't put it past Sirius to turn into a dog with the intention of freaking me out! And then he'd drool all over me and find it hilarious :-P

I admit, it would be quite amusing in public to present a stunning immortal as your BF. But then I'll get undressed and find it totally unfair that he is prettier than me *is weak* Plus, at some point people will start to wonder why the guy still looks the same after all those years, and you'll have to get rid of him anyway. And then get a new one - oh, the hardships!