Throughout our lives, literally from babies into adulthood, it is assumed that you are/will be straight. Does any new mother meet another mother at the park and say "Oh, your son is so cute! Let's call him my son's boyfriend!"? We ask little girls about their ideal prince charming, teach kids all about how to have safe sex to avoid pregnancy, and of course, just generally ask people personal business like "So, you got a girlfriend?" Even people who might fit into straight society's stereotypical view of gay people are still assumed straight most of the time, because it's considered an insult to straight people to be assumed gay, as though being assumed straight is a compliment. It's not a compliment. I had to go through all the aforementioned bullshit, plus coming out and accepting myself, and that generally adds up to a pride in who I am. When that part of me becomes invisible, I become invisible. There are ways to combat this, such as coming out in casual conversation almost as soon as I meet someone new, but that doesn't really stop the overall problem and perpetuates the constant coming out of the closet that adds stress to everyday life.
Gay people are gay. By saying, "Oh, you don't seem gay" or "You seem too masculine to be gay", you are not complimenting me. Gay people can be masculine or feminine or somewhere in between, just like straight people. There is no one clear way to tell if someone is straight, gay, somewhere in between, or somewhere outside of it all, unless they straight up tell you, so please stop assuming.