'Shahs of Sunset" Star Weight Loss Secret Just Having Tons Of Sex!

Mercedes "MJ" Javid swears she hasn't been photoshopped, she really did drop 20 lbs., and she tells TMZ ... she did it the old fashioned way -- lots and lots of sex ... and some traditional exercise.

The "Shahs of Sunset" star says she's seen all the haters accusing her of doctoring the pics from a recent swimsuit shoot -- but says her real secret is simply banging the weight away.

Best of all ... MJ claims the more pounds she shed, the more sex she had -- and the more sex she had, the more calories she burned. The least vicious cycle ever! 

She admits cutting back on hard partying and late-night boozing definitely helped too ... along with hiking, biking, tennis and dodge ball -- but says she's definitely sticking to her sex diet.  She says bluntly, "I'm too pretty to be fat!"

Her ego is NOT slimming down.