(I Am Not A Lawyer)
I strongly suspect that it would be highly unlikely you could be charged with any kind of obscenity for consensual sexual, even outdoors, IF a person had to trespass on your property to observe you.
In that regard, it is often necessary by law to post your property to be able to claim trespass occurred. You may think Cap'n Jack's signs are snarky, but the legal principle is sound. Check you state's statutes for any specific requirements for the signage's size, wording, and placement.
I suggest this one:

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