Why is s/he telling me? ... I'd prefer not to know.
This may be your reaction initially. You may feel let down or hurt, that all of those aspirations you had for them are now apparently gone.
Amidst the barrage of emotions, it may be a good idea to take a step back and realise what, fundamentally, has happened. Your child, in their vulnerable state, has disclosed something intensely personal to them, and is entrusting this knowledge to you as their parent because they do not want to lie, and live an unhealthy double life. In short they are saying, “I love you Mum or Dad, and I respect you, and want to include you in my life”.
Try to put yourself in your child’s shoes. Try and imagine how difficult it would be to be unable to talk about who you are attracted to, how you feel about a certain person, or even where or with whom you socialise. To live a lie and ignore your sexuality, such an integral part of you, is emotionally damaging, and isolating. Try to give yourself time to come to terms with this news. It has taken your child X amount of time to come to terms with their sexuality; so naturally it will take you time to come to terms with it too.