In Canada, "Six Feet Under" is carried by TMN - The Movie Network (a commercial-free, pay TV channel). I don't subscribe to this pay channel so I haven't seen the series. I understand that it's supposed to be very good.
Gays on commercial TV were en vogue last year mainly because advertisers have discovered that gay people, for the most part, have high disposable incomes. Bring gay and gay-friendly viewers to a TV program and, hopefully, these viewers will see the advertisers as gay-positive and buy their products.
It's great that commercial US TV is starting to reflect gay relationships but it's really because advertisers have discovered this market and their money.