Uh, the porn world is a very very, VERY insular one, that doesn't exactly roll out the red carpet for strangers, reporters or fans. There is literally no chance of you getting to visit and hang out on a porn set unless you are actually invovled in the production is some way, either as a performer (yeah right) or camera/sound person. There also aren't any clubs*/gyms/hangouts that cater exclusively or even semi-exclusively to porn stars and even if there were, again, you'd have no chance of getting in unless you were in the industry.
AFAIK there aren't any porn expos or conventions in the LA, the big ones are always held in Vegas.
I suppose you could try hanging around the Hustler store on the Sunset Strip in LA, but you'd probably come across as really creepy and pathetic more then anything else, which tends to turn off the chicks in major way.
*I forgot about strip clubs. You could at the very least ogle some naked chicks there and maybe you'd get lucky and some porn chick would be touring there that week. your best bet would be to call around to all the strip clubs and ask if they have any porn stars coming through.