Cold blooded A. is a old and VERY inaccurate term, reptiles CAN regulate their body temperature and are not fully dependant on their environment and B. strip a human of his clothes or a wolf of his fur and they would freeze to death as well, does that mean they wouldn't be able to survive in Skyrim?
Plus being a fantasy race it's easy to assume they can fully regulate their body temperature.
A first i got to blow your theory about the reptiles in real life as i got here a couple of sites you may wana read
http://coolcosmos.ipac.caltech.edu/image_galleries/ir_zoo/coldwarm.html
http://en.wikipedia.org/wiki/Poikilotherm
http://animal.discovery.com/guides/reptiles/basics/humidity.html
and maby you readed thise piece : One of the more fascinating reptile facts has to do with chameleons. They're usually thought of as lizards who change their skin color and texture to suit their environments. What's more accurate is that they will change their skin color or intensity and brightness according to their mood; or in their "attempts" at regulating their temperature.
from thise site : http://www.lizard-landscapes.com/reptile-facts.html
now i must say i dont know all of the of the reptilians
but in Commun few basicly we consider most repilians "cold blooded"that they "Can't"regulate their own body temperature
now its tru some anfibians or reptilians got enzimes in their body with a anti freeze prefent then to freeze to death but go in hybernation instead
but they still need the sun or other heat sourches to warm their body temperature and shadows to cool it
now for argonians i know their not 100% reptilian and that they can use magic items to keep them warm
but you also se lesser agronians in colder regions as i belief they disslike the cold
now thise are just my few on the things you can argu with it what you want
ps sorry about my bad spelling but thats not the argu of the topic here