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.