Appropriate?

Is it ever appropriate to slap someone on the butt?



I know within a sports setting (i.e., football, baseball, basketball, etc...) it would be cool and no one would think anything of it. But when you play tennis with all gay men, and a guy that you have know for a brief while (and WASN'T on your team) slaps you on the behind...it that okay? This guy seems to always to be staring at me, with lust in his eyes, even when he is with his alleged boyfriend. [I only say alleged but I have only heard in passing that he was coupled with someone that belongs with the tennis group.]

Am I overthinking this, or should I be concerned?