Jennifer Lopez & Casper Smart -- Not Dating, We Swear ... But Still Going Down South

Jennifer Lopez and her NOT boyfriend are enjoying a NOT vacation in Mexico for his NOT birthday.

You get our drift, right? It IS Casper Smart's 28th birthday, and he and JLo ARE on vacay with her kids -- yet during an interview this week ... Jennifer insisted she's single.

Casper did some jetskiing ... JLo's kids went horseback riding, and JLo followed them all around town. Check out the pics ... no kissing or canoodling, but they're definitely close.

As we reported ... sources connected to Jen and Casper tell us they never really broke up -- and a couple weeks ago they were kissing outside "American Idol."

But yeah, like she says ... they're not dating.