Simon Cowell Begs Photogs ... No Marriage Talk in Front of My Baby Mama, Please!!!

Simon Cowell got put on blast when a paparazzo dared to bring up the "M" word while Simon's girlfriend, Lauren Silverman, was feet away -- and you gotta see the ex-'Idol' judge's reaction.

Truth is ... this clip is weird all around. Cowell was partying at Ago in West Hollywood after the "American Idol" finale, but did NOT seem like the guy America came to love/hate.

He talked about crappy old 'Idol' wannabes, replacing Howard Stern and even baby diapers -- and not ONE negative word from the guy ... until the whole marriage thing came up.