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YouPorn ﻿is now all about the D -- as in, Dunkin' -- because it's getting revenge on Starbucks for blocking Internet porn in its stores ... TMZ has learned.

Reps for YouPorn tell TMZ ... the porn giant is now strictly pushing Dunkin' coffee on its workforce. The company's VP sent a memo to employees announcing a ban on all Starbucks products from YouPorn offices effective New Year's Day.

Point is ... 2 can play this game.

Starbucks just announced it's blocking customers from watching XXX material while on their free WiFi, this after mounting pressure from the Internet safety group, Enough is Enough. YouPorn is simply responding in kind with a blockade of its own.