Your question is based on the premise that Bruce and Betty are still together; the most obvious and likely answer, based on the facts, is that they aren't.
Bruce left Betty at the end of The Incredible Hulk, when he ran off to his remote hut to try and gain control of the Hulk part of himself. Since then, we haven't seen any indication that the two of them are together. In particular, there have been several situations where other Avenger's significant others were brought up, where it would have made sense for Betty Ross to also be mentioned:
- In The Avengers, Coulson makes a point to say that Jane Foster has been taken somewhere safe, for Thor's benefit
- In Age of Ultron, both Thor and Tony mention their respective girlfriends (likely to explain why they weren't in the movie)
In neither case does anyone ever mention Betty Ross, which to me stands out as a rather notable omission.
Based on his actions in Age of Ultron, especially
his conversation with Natasha in the Barton house, where he lists all of the reasons he can't ever have a long-term relationship with a woman
it seems very, very likely that he left Betty "for her own good". (This is a classic part of the superhero's tragic backstory: they get to save the world but they don't get to have a normal life.)
