The legendary sports broadcaster has been doing some fun charity work this month ... providing play-by-play of fan-submitted videos on social media in exchange for good-cause donations.

Buck's voiced over some pretty wacky clips -- a guy making chicken wings, some dude throwing darts and even marble racing were all part of his package last week.

But, Joe said in an interview with KMOX on Monday ... some of the vids fans are sending his way are straight-up NSFW -- and he'd appreciate if people stopped doing it.

"I've had a couple of submissions from let's say, a man and a woman, that just didn't seem appropriate to put my voice to in this stage of my life," Buck said, "Maybe later in life, but not now."

Joe says people shouldn't try to sneak it in behind seemingly normal videos either ... 'cause he claims he's going over even the benign videos with a fine-tooth comb!!

"I look at these videos very carefully and pick the ones that seem the most wholesome to put my voice to."