They should have used younger actors.
Their performances were fine, but both Oleyowo and Pike are around forty years old, and the characters they play were in their mid-twenties when they married.
The film would have been a lot more poignant if the people dealing with the consequences of an impulsive decision were very young, young enough that you can believe they'd marry without considering how it would affect the big picture. Sure, people of forty do behave that way on occasion, but they're less sympathetic than the young when they do. They're old enough to know better, after all.