Certain acts used to be considered "tabu" in porn-- but then they became normalized and commonplace. For example:
INTERRACIAL: Used to be a "big deal". White girls who did it were few and far between. For me, the breakthrough star was Connie Peterson in the late 70's-- (Little French Maid and others). Seeing a petite, youngish blonde with two black guys was utterly spellbinding.
ANAL: Fairly niche, became more and more common in the 80's. Like interracial, some girls did, most did not. Then came DP's. I remember seeing my first DOUBLE ANAL scene with Brittany Stryker about 1985. Amazing! I hadn't even realized such a thing was physically possible.
CREAMPIES: You rarely saw 'em. The male completed on her face/ on her stomach/ or on her ass. Started picking up steam in the mid 90's.
SALAD TOSSING: (female licks male's ass). You started seeing in in the early '00's. Became pretty mainstream. Probably a bit degrading, but there's enough playfulness to it that it can pass for something the woman enjoys. Still handled awkwardly, though. Almost never VERBALLY acknowledged. It's rare to hear a man say "That's right. Get in there and lick that ass" or a female to say, "Do you like how I lick your ass, baby?"
PISSING ON/ PISSING IN HER MOUTH/ PISS DRINKING- It started with the former, then has evolved into the latter. Not my favorite- but interesting. Probably the ultimate male-dominance power move. But does any WOMAN actually enjoy this?

SO WHAT WILL BE NORMALIZED NEXT? (I mean-- there's not much left!)
Three predictions:
- The woman rectally penetrating the man during intercourse, finger-fucking him. It's out there-- but is ultra rare. First female I saw doing it was Sasha Gray. There's at least one more, but the name of the talent escapes me.
- The "C" word. You rarely hear it from either participant, but it really spices up a scene.
- White women using the "N" word and/or racially taunting her partner. Shane Diesel's stuff used to make a big deal of this-- but then it faded. The current fashion seems to be the woman making no verbal reference to race of their partner. Considering the cultural sensitivity concerning racial matters I wouldn't be surprised if there's some sort of industry standard or unwritten rule concerning this.

Your comments/contributions are welcome.