Yes we do
Children cannot give informed consent to sexual activities, so, that’s accounted for.
What? You said fucking in public is okay as long as everyone consents, if children aren’t capable of consenting, like you said, how is fucking in public okay?
In general, it is not. That’s what I mean. Anyone in the potential “audience” must be capable and willing to give informed consent to participate in a sexual act. If that condition cannot be met, the act is not ok.
ETA: My intent is to apply more generalized ethical reasoning to the situation. Sex in public is not, itself, fundamentally wrong; forcing others to participate in sexual acts without their consent is. This defuses avenues for kink shaming, sex-negativity, and other similar lines of reasoning.