Is it safe to use my pets as living room furniture pre-heaters?
It's probably safe that you use your pets as living room furniture pre-heaters.
Yes, they warm up the furniture so they don't freeze outside.
It is good to not let your pets freeze outside. Some people don't like when animals are on the furniture because they leave fur everywhere.
I put a blanket on top of the furniture.
I really appreciate that you treat your pets like family and allow them to rest on your furniture, many people do not and simply treat them like wild animals.  Your okay in my book.