When you say “let,” it sounds like you are the one in control of son/daughter. This makes me think that they’re not a minor, and this is illegal in the porn industry.
If they’re not minors, but still living in your house and you’re against the idea, then I wouldn’t allow them to live in the house while in the profession. They’re 18+ and they get to decide what they ultimately want to do, especially if it’s legal for them to do it, but it’s your house and you get to decide what kind of tenants you want living there.
If they’re not minors and they’re not living with you, then I don’t think you really have the right to force them to not follow that path. But I think you should be allowed to voice your opinion against the idea (if you were), and hope that your son/daughter would see your side of it.
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if you’re asking for my own personal opinion of what I would do in the situation, it would depend on how old they were. If they were just turning 18, I would strongly influence them away from the situation. If they were in their 20s-30s, whatever.