Okay, I'm just going to try and set this straight (haha!).
Yes, a person's sexuality doesn't define them entirely. But sexuality makes up one component of an entire identity, and each component like your age, your beliefs, your ethnicity, where you grew up, your education, and so on, all together shapes who you are as a person.[/quote]
Now, as a heterosexual person, maybe, they won't necessarily respond or interact the same way a homosexual does. There might be similar reactions when you get down to the core of it, but there are a lot of nuances people tend to not notice. You see in movies that you have a main character that brags about scoring a date with the pretty girl next door to his pals after a long day at high school. In this day and age, if the guy snagged a date with a pretty boy, he might not be so open about it.
So, she might comment about meeting a nice girl over a nice boy. This helps define her...how? How does them stating "she only likes girls" change how she might approach relationships? It seems like something for the player convience so folks can't fight about whether a character is "really" bi ala Anders. Otherwise, gay, bisexual and straight people do react largely the same way, so I'm not sure where you got that from.
So what I'm trying to say that true, sexuality doesn't underpin a person's character totally, but it influences how they think, how they approach people, and how they communicate. And sexuality is all the more insidious than race or age or sex because it's not as obvious as those three, so it does get confusing just how much sexuality affects a person's thought patterns, or discourse, or who they relate to, and so on. Thing is, it just really depends on the person as well.
Personally speaking, sexuality is something that's very personal to me, so it does influence other areas of my personality- this guy is attractive, I'm more sensitive to discrimination based on gender identity and orientation, and playing my cards really close to my heart because society in general isn't that great when it comes to non-heterosexuality despite nowadays communities becoming more socially progressive. But that's not all of me, because there's other areas that play into it as well, like my age, my education, growing up in a western English-speaking society, my race, and so on.
"I still wish I could ask them in person how does a character's sexuality fleshes them out more?"
I only hope I answered this question as best as I can.