I don't know how to answer this. Mostly because it wouldn't ever happen. My kid would never be given a reason to not have me know, so there would be no coming out, and if it was something they suddenly discovered one day, there would still be no reason to say anything. Bring a girl home, bring a boy home, to me it is just bringing a human home.
As for others coming out, I also wouldn't really know what to say. I mean if it was difficult for them, I'd reassure them, but as for me, it essentially amounts to someone saying that they like to sing in the shower or don't like broccoli. Be happy, be in love, I'll leave the obsession with crotch analyzing to the barbarians.