My penis doesn't get hard when im about to f*ck or it gets semi flacid. i'm 21 years old, im a body builder i eat correctly high protein etc?

See a doctor. And by that, I mean a psychiatrist.

If you are injecting steroids, that is the reason.

may be from watching too much p0rn

6 inches is well below average and so you are probably experiencing a great deal of shame and self-loathing. This would explain why you become aroused when someone physically harms your genitalia.

Do you wake up with morning wood (full erections) or have them in the middle of the night? If yes, then it is not a physical issue but a mental one. May be stress or apprehension when you go to have sex, often called performance anxiety. If you don't get full raging erections at any time like when waking up with one or during the night, then you may have erectile dysfunction and need to go talk to a urologist about that.