YTA. I don't blame your girlfriend for being pissed.  You grossly implied that she is being sexual with her dog, when she isn't. You're also telling a grown woman, who isn't harming anyone, how to live her life and what to do in her own home. 

CONTEXT MATTERS.

Nudity is not sexuality.  Being naked isn't sexual until you make it sexual.  I bathe my dogs in the shower.  I do it naked so I don't have to deal with soaking wet clothes.  Is it sexual just because I'm naked?  No. It's just easier and more comfortable that way and the dogs don't give a damn or even notice the difference whether I'm wearing clothes or not.  Sometimes, when I sleep naked, my cat will sleep on top of me.  Is it sexual just because I'm naked? Obviously not.

"the dog in my spot on the couch, asleep with her head on my gf's naked thigh. That's just so weird? How is it not?"

How is it weird? You are reading too much into it.  You said the dog is pretty much like a little kid to her.  It's not unusual for mothers to change clothes, shower or otherwise be naked in front of their babies or young children.  Do you view a mother breastfeeding her kid as sexual too? Just because you're inhibited and think nudity automatically equals sex, doesn't mean the rest of the world feels the same way.