It's a pretty well known fact that Roman Reigns is married and has children and that Charlotte Flair is also married, so if WWE tried to go with that storyline, they would be exposed thanks to Social Media, just like they were exposed years ago when they tried to pretend that Kofi Kingston was of Jamaican Ancestry, when in fact his Family was from Ghana. WWE figured that WWE Fans could find Jamaica on a map easier than Ghana.
As Kye states, Seth Rollins and Becky Lynch have known each other for a long time before this romance went public. As WWE has shown time and time again, they have no problem exploiting real life relationships. ,