What the heck man?
It's really cool that you've got it all figured out and we're all seriously happy for you - and we support that! No one should feel guilty or scared because of their sexuality, but unfortunately so many of us still are, due to the way we grew up, things we were taught or people we were around.
But what you wrote is so narrow-minded!
"If he's over 18 he's had plenty of time to figure things out", seriously? Do you believe that, like, by the age of 18, you gotta have yourself figured out? That's so wrong, because 18 is so young and you're basically just starting to explore life freely. Like, I believed I was asexual until not so long ago and it took me so much time to accept that and to stop trying to force myself to change. Only to realize in my middle twenties that I'm, well, not so asexual. And I'm STILL not even sure where I fit exactly.
You use empathy to put yourself - to imagine yourself - in another person's shoes and see things from their perspective. You don't have to have knowledge of something or experience it to be able to use empathy.