The next time this happens, blow your airhorn. The fact is, the "lovers" will become extremely uncomfortable and go away. They don't want you to see them, (one of them might be married) they were being airheads, obviously.
A little public shaming goes a long way.

Reminds me of something that happened in the early 1960's when i was an 11 year old girl. I was in the park with 4 other girls and I was the oldest one. A man approached us and exposed himself. We all started laughing derisively and pointing. He slunk away.