I actually have what I think is a vaguely similar situation to the OP in a series I'm writing in that I have three major characters in an adventure series: one man, one woman, and one silverback gorilla. As the series has progressed the gorilla has married his childhood sweetheart (also a gorilla), the woman has found love with another woman, and the man has avoided any romantic entanglements.

The only thing I had planned out before I began the series was that the woman would not be the man's love interest because I wanted to ensure she wasn't relegated to an appendage. It's actually worked out well for me as most people have told me that they loved her story, and one absolutely adored her and her girlfriend in the adventure after the one in which they got together.

My only suggestion is that I found it very helpful not to define anything about characters' sexuality or relationships until it came up in the story so that might be worth considering as an approach.