I would argue that sexuality only informs one's personality where it changes one's life experience. For instance, in a world where homosexuality is not stigmatized, growing up gay would arguably be no different than growing up straight. A companion with a backstory involving an old boyfriend could as easily have had an old girlfriend, and only that single term need be adjusted.

Our perception of a character might change based on their sexuality, but their own perception, or that of other characters, might not.