Eva Longoria -- I'm NOT Pregnant with Mark Sanchez's Baby

New York Jets quarterback Mark Sanchez is simply not reproducing ... TMZ has learned.

Despite reports that he knocked up his GF Eva Longoria, the actress' rep tells us, "Rumors of the couple being pregnant are false and absolutely not true."

Still, sources close to the couple tell us Mark is "truly serious" about his relationship with Eva ... with one person telling us it already seems like "true love."