For many parents, the first reaction to hearing "Mom, Dad, I'm gay" can be denial. It's tempting to dismiss what your child is telling you as "a phase," or "just experimenting," or plain-old teenage confusion.
If that's what you're feeling, keep it to yourself. Respect your child enough to treat his or her feelings as valid. After all, it may have taken months or years for him or her to get up the courage to tell you this secret, and that's plenty of time to reconsider if confusion, experimentation or a phase is the underlying cause.
Validating is the first sign of respect. The next one is letting your child set the pace. Just because your son or daughter comes out to you doesn't mean you should tell your best friend or write about it in your blog or, however well-intentioned, wear an "I Love My Gay Son" T-shirt to the grocery store. Respect your child's privacy. Sexual orientation is a personal subject.
It's also a controversial one, and many parents experience feelings of guilt when they find out their child is gay. That's where education comes in.