From the sounds of it, I would probably say Bisexual. However, some points to consider:
- Do you think of the male characters as identifying as homosexual or bisexual? I assume the latter if the third part is Group Sex. This doesn't, in itself, decide the category (I'm planning a Gay Male story where the main character is probably no more than a little curious) but if you were viewing it as these men finally embracing their homosexuality and beginning this new chapter of their life, then probably put it in Gay Male.
- How much of a part do the marriages play in your story? If these relationship feature heavily, then Bisexual would probably feel more right. If it were just mentioned in passing that these two men were married and then you plunge right into some hot and heavy man-sex, then put it in Gay Male.
- What readership does it appeal to? Most regular readers of one of these categories don't necessarily read much of the other. From what you've told us, your story would probably appeal more to readers of Bisexual stories, especially where they might be a little curious themselves. You are the best judge of that, of course.
Tags (although I'm not a huge fan, in general) can play an important role in situations such as this.
Good luck with the stories. I'm giving myself similar challenges (when I get the time). I'll try to give them a read when they get published.