If tons of kids at school used racial slurs or sexist comments would it make it any less insulting.
A word is a word, and you can defend that words hold no weight if you prefer. You can also claim to not get offended, if you're LGBTPQ or not. But when people use these words, there is a tone to their voice. There is meaning. And usually, with the "gay" comments, their meaning are meant to be hurtful and insulting.
When a person says, "You're so gay", the the phases can be taken a number of ways. If a friend calls another friend gay, because the gay friend is acting or dressing in a stereotypical way, then it is still insulting, but in a friendly, joking way.
But the reason people get so sensitive about the "you're so gay" comments are because what their deeper meanings are referring too.
In the cinema you will see the social stigma placed among homosexuals and popularity. The stereotypical sports coach will pump up his students, or put them down to encourage results, by calling them "girls", or "sissies", or "f---ots", because this is meant to make the boys feel emasculated, weak, and undesirable for their lack of masculinity.
To be gay (girl x girl / boy x boy / etc), it is generally looked down upon as a thing to outcast you; esp. encouraged in schools by children/teachers who are taught from tv, parents, friends, and other children that being gay is wrong, perverted, as well will earn them a ticket into a personal Hell (as dictated by their religious beliefs).<-------(quite frankly, I believe the hatred of homosexuals in religious texts are the most impacting on their degradation; since most people take the words of their holy books are fact)
Since a majority of countries are followers of religions where homosexuality is labeled sinful and wrong, it is hard to not understand why when someone is called "gay", jokingly or in anger, it is because others are trying to lower you, make you feel bad about yourself (even if one is gay or not, calling them gay is meant to hurt their feelings), and feel less like a person of value.
So to many, and many others, the "that's so gay" comments are an insult.
Can you imagine waking up as a gay boy, brushing your teeth, eating breakfast, going to school. Normal, normal. And then having to be bombarded with comments from passersby, "queer", "c----sucker", "--g"; shoved into lockers; tripped; lunch spilled; money taken; beaten up. And then by gym class your coach yells at you to emasculate, and the other boys judge you and make hurtful comments. Then you have to go home, try to cope with the fact that yes, you're gay, and try not to fall apart because you have school again tomorrow.
Homosexuality may have always existed, and there may be a lot of them, but homosexuals are still a minority. Because of prevalent religious morale, they will always be subjected to these pains. So when someone passively says, "you're so gay", the meaning doesn't stop after the 'y', it's just more weight to add to your shoulders of the pains you suffer for being who you are, and not submitting to social pressures, and lying to yourself.
This may sound melodramatic but it's how it is, at least from what I've seen and felt.
And being able to shrug off the comments, ignore them, that's a good thing. It's just hard for many others.
Edit: Additionally. When someone says "that's so gay", they don't always refer to a sexual act of homosexuality, but use the word "gay" as a versatile replacement for uncool, idiotic, stupid, dumb, etc. Even using the word to replace another, ignoring the true meaning, is equally insulting. It says on a subconscious level that gay = bad/ stupid.