First, any relationship softens the character by humanizing them.

Look at breaking bad - the whole story is about Walts adultery - to building an empire instead of his family.

Had he suddenly had a love interest (male or female), that would detract from his "true" love of building an empire against his love for his family.

The Barlow relationship was introduced first, and was a clear enough motivation for Flint to become what he was - hell bent on chaos against the British empire, exploiting his crews love of gold to get there.

The twist was from out of left field, and (to my recollection) wasn't really spoken of afterwards. It didn't push his character forward, and it "softened" him by diluting his motives beyond piracy and vengeance.