Naomi Watts
Naomi Watts has been in blockbuster hits like King Kong, The Ring, and The Impossible. At the peak of her career (aka, pre-children) she was constantly booked. It would make sense that she hadn't gotten into a terribly serious relationship... Until she met Liev Schreiber.
Watts and Schriber were together for 11 years. They even had two kids together, Kai and Sasha. They never got married, which was probably a smart thing to do since they broke up. Watts has been with Billy Crudup since 2017, but if their relationship is anything like her previous one, then marriage probably isn't in the future.