My guess is they are not a couple based on the following facts.
1, Where's the beef? Apart from endless hype nobody has caught Kristen or Rob sneaking around together. With the paparazzi on them 24/7 such a shot would surely have surfaced.
2. Rob has clearly had close encounters with several other young ladies --remember the trysts in the tavern story from England -- well it rang very true.
3.They have both denied it. Funny that, we won't take at face value what they have said themselves about their relationship -- professional but not intimate.
4. Bad career move. Rob's entire legend is base don him staying single and allowing girls to fantasize about nabbing him. Happily engaged or married Rob does not cut the mustard.
5. Common sense, they are both way too young to be settling down.