Pretty soon, it's going to be hard to find a single white-white couple on television.
OK, let’s go through a random list of TV series (i.e. ones I watch regularly) and see if you are right:
Game Of Thrones: many, many, many relationships, but as most characters are white, most of them are “white-white”.
Person Of Interest: Reese never had the chance to start a relationship with Carter, every other relationship I can think off the top of my head is white-white (though I don’t think you would count Shaw/root) and two black-black.
Castle: Of the regular cast one relationship is Hispanic/black, the rest is white/white, especially the main protagonists.
Supernatural: Almost all relationships are white/white, if you count werewolves, demons, angels etc. as white.
Buffy: I am only at the start of the third season, but until now we have seen only white/white (again, if you count werewolves and vampires).
Penny Dreadful: Again, only white/white.
The Big Bang Theory: I only watched until the 7th season, but until then the three couples among the main cast are all white/white.
Daredevil: White/Hispanic, though everyone knows Matt will end up with Karen Page, Wilson Fisk/Vanessa are again white/white.
Firefly: The longest relationship is black/white, the rest seems to be only white/white.
Agents of SHIELD: Two Asian/white, one Asian/black, one white/white
Sherlock: Holmes/Adler is white/white, John/Mary is white/white, but I think one of John’s many girlfriends was black. And of course, John/Sherlock is white/white.
Doctor Who: In NewWho the only mixed-race (at least among humans) I can think of is Rose/Mickey (until she dumped him for the alien with two hearts and PTSD) and Clara/Mr. Pink.
Hm, in contrast to your statement, there still seem to be more white/white relationships than anything else. Who’d have thought
PS: Oh, and advertisers will put in their ads whatever will make them more money. What do you have against capitalism?