What's so special with being gay? It's normal and part of everyday life. People are either bi, straight or gay, that's life what's the big deal?
The big deal is that being gay or bi is a minority. A big one (if that's not a contradiction in terms). And one which has historically (and still to this day) elicits a range of prejudices from everyone from parents to church leaders to politicians. Society, generally, is set up by and for straight people. Everything gay people have now, any rights and protections, we've had to fight for. Gay people (like me) grow up being told, explicitly and implicitly, that being straight is normal and that anything else is wrong or unwelcome. This makes it very difficult to come to terms with in your own head. Quite often, a gay teenager will feel very isolated. They may well be the only person within their social circle who is gay. So there are limited frames of reference.
So the 'special'ness of being gay is that it is a) rare and b) something that means you have to define yourself against what society, and quite often your own family, expect you to be. This can feel like everything when you're 16, frightened, isolated and lonely.
I have told my dc since they were pre-teens that some people prefer to be with people of the same sex and that's totally ok and that if they should decide that that's right for them when the time comes then that'd be ok and we'd be completely supportive as each person makes this choice for themselves.
You're making a big, and really quite offensive, error here. Being gay isn't a choice or a preference; it's simply who someone is. It's as fundamental a part of them as the colour of their skin. It's not something they choose, prefer or decide.