When the day comes that I walk in on my son masturbating, how do I let him know it's normal and good without it being really awkward?
Why wait for that day to come? Instill in him a respect for his body and respect for privacy from a young age. When kids discover their body and what feels good, you absolutely should be telling them what is appropriate in public versus private, but never shame them. If your son has his bedroom door closed, knock first. Encourage him to respect other people's privacy and lead by example by respecting his. Embarrassing moments like walking in on masturbation that are accompanied by shame will only tell an impressionable child that there is something wrong with them or their body. Bodies are not dirty. Masturbation is OK. Sex is OK. Set realistic boundaries on things like that from the get go and you can hopefully avoid weirdness, resentment and lifelong hang-ups about sex. There are even children's books, like Cristian YoungMiller's Everybody Masturbates and Everybody Masturbates for Girls, which tackle the topic in an age-appropriate way. I know some people will disagree with my advice, but I would rather live in a world where we no longer feel ashamed of our bodies and natural desires than whether a few stuck-up prudes get mad at this blonde bombshell. Body and sex shaming has caused enough harm. The revolution is in our own hands! Good luck.