@cancerino
(I know this is somewhat of a late reply but it's worth answering your question): in the first chapter it's pretty explicitly stated that he saw her as a girl when they first met, and that they'd been dating for a while before he even knew she was trans, so it's pretty safe to say he's straight.
Secondly, you may see that trans people really, REALLY want to drive home the phrase "trans women are women", this also means that "men who are attracted to trans women" are "men who are attracted to women", and hence straight, while to some it may seem minor, from the viewpoint of the people involved referring to a man as gay for being with a trans woman also implies said trans woman to be another man, which can be perceived as a denial of their own identity and effort in transitioning