I think it depends on the models and the shots/angles.

If the models are attractive - hot bodies, big dick, cute face, etc it helps. Also if they are "into" the scene, and are good porn stars. I find amateur/straight porn is not nearly as erotic because the guys have no clue what they are doing and there is no passion/attraction.

Lighting, camera angles, and shots are really important to. Showing an equal amount of face shots, panning, close ups is really important at creating the mood/fantasy and emotion of the models. If the shots are mostly of the face, or full shots you don't get to see all the curves and body parts that you would if you where doing the "real thing". To much lighting and the scene looks fake, to little lighting and you cant see anything.

That's my two cents.

D