I've seen two extreme reactions to a child coming out to a parent. We've all seen the ugly, horrific, and abusive responses, which I refuse to link here, because they devastate me to my very core on behalf of the child. We've also seen the opposite that goes something like this:
So, you're gay. Cool. Can you pick up some milk on the way home? Love, Mom
While this is quite clearly preferable to the former, it's still not ideal, despite the props it gets from those in my camp. No, I don't think this is the way to go either.
If your child has taken the time, made the effort to make an actual "coming out" statement to you, it's clearly a big deal to them. There's a reason she is coming to you to make a point of having this conversation. Maybe she heard horror stories of coming out from her friends. Perhaps she's seen the same awful videos we all have and it messed with her head. It could be that you've made an off-hand comment or two without even considering how it affected her as she silently listened from the other room. Whatever the reason, she's concerned about coming to you enough to actually make an event of it.
If all this is the case, you cannot simply dismiss it. Your intentions may be good. You may think to yourself as you're hearing her words, "This is no big deal. You don't want her to feel bad. Don't make this a problem," so you swing too far and rather ignore her emotional declaration. Again, this is a dismissal, regardless of how pure your intent. She has feelings. You cannot brush them aside.
No, being LGBTQIA+ is not an issue, but a child's feelings always are and so the proper thing to do if a child comes out to you is to acknowledge said feelings, care for them, offer an open ear and heart to listen to them. Offer a hug, show your love, never skip a beat, but certainly, certainly do not dismiss.
I think the ideal situation would be that you and your child have developed such a close and open relationship over the years that there is no one specific coming out talk, rather they chat with you and you with them when feelings arise of all types, including sexual feelings or feelings about who they are inside. In that case, this blog post likely isn't for you. This is for those whose children aren't as comfortable for whatever reason and do have to screw their courage to the sticking point to say to their parents, "I'm [insert who they tell you they are here]." More than that, even, this is for the parents whose children have that conversation with them and who love them unconditionally.