they don't mean you in particular for being unable to understand how the MC feels and how comfortable he is with his sexuality, everyone here is glad you have yourself figured out and we hope you stay in a situation where you can be yourself openly, but you are not every gay person. a lot of gay men and women don't realize they like the same gender until very late in life because of how they were raised or who raised them or who they were raised around. back when I was a minor I constantly thought that I knew nothing about myself because I was a minor and that what I was interested in might change when I grew up and or I might be straighter than a needle, spoiler alert, I never did, and also on the main topic about why most M/M games have "straight" MC is so it can relatable to those who still don't know themselves and wouldn't mind exploring via real life experience, or virtual ones. some games ignore it because it's not everyone, some games add it because it's not everyone.