The Internet has been good for cable companies. Comcast (NASDAQ:CMCSA), Time Warner (NYSE:TWX), Cablevision (NYSE:CVC) — they owned the cable, so when it was high-speed Internet’s time to replace dial-up technology more than a decade ago, they opened up a whole new stream of revenue. It’s been great.

At the same time, all cable companies — and even other premium television services like DirecTV (NASDAQ:DTV) and Dish Network (NASDAQ:DISH) — have got to be missing the way it used to be back before most U.S. households had easy access to the World Wide Web. Those were simpler days, when the money flowed like an affordable but delicious boxed wine. Those were the days when cable companies made their fortunes on pay-per-view pornography. Those days are gone.

An article in The Wall Street Journal highlighted the fact that premium television services are watching quarterly revenues sag on what DirecTV in particular called “lower adult buys.” Both DirecTV and Comcast noted declines in adult film revenues from their television businesses, though they decline to specify dollar amounts. Time Warner lost $14 million from its total on-demand revenue because late-night audiences simply aren’t interested in paying for dirty pictures any longer. Playboy‘s (NYSE:PLA.A) TV endeavors have seen earnings practically halved, with Playboy TV and Spice bringing in just more than $44 million in 2010, compared to almost $76 million in 2007. Analysis firm SNL Kagan said that overall revenue from on-demand and pay-per-view porn dropped from $1 billion in 2008 to $899 million in 2010, and it expects 2011 to be flat.

What happened to the good ol’ days when premium television providers could rely on late-night audiences to plunk down $10 for a half-hour of programming involving sorority girls and possibly nurses misbehaving? An unnamed cable executive told the WSJ that the problem is the overall devaluation of television content by “alternative models” — namely, the proliferation of streaming video options online.

That opinion misses the point entirely, of course. While the Internet clearly is changing how television is monetized, it’s certainly not devaluing it. If alternate models are devaluing television, how has Netflix (NASDAQ:NFLX) grown so much during the past two years? If the Internet has devalued anything, it’s pornography itself. What was once an expensive and restricted commodity is now commonplace and free everywhere on the Web.

If cable companies want to keep a revenue stream flowing by preying on their audience’s more illicit tastes and desires, they’re simply going to have to rethink what people badly want to watch after midnight. Follow what Scripps Networks Interactive (NYSE:SNI) has done with the Food Network, for example. Start making half-hour-long clip shows of television chefs putting the finishing touches on signature dishes and call them “Hot Co-Ed Barbecue.” Another alternative: Fantasy news. Hire Dan Rather to read fake news broadcasts about how the economy is booming, the U.S. government is stable and making progress while returning to traditional values, and the world is at peace though less prosperous than the United States. Just rethink what a fantasy people would pay to watch might be!

A more realistic proposal on recouping revenue lost to declining cable porn sales: unlimited access to free on-demand programming on connected devices like smartphones and tablets. Cable companies can add an additional fee on top of existing cable packages. Time Warner and Cablevision already are experimenting with this by releasing streaming TV apps for subscribers on the iPad. It’s time for television providers to find new fortunes in those pesky alternative models.

As of this writing, Anthony John Agnello did not own a position in any of the stocks named here. Follow him on Twitter at @ajohnagnello and become a fan of InvestorPlace on Facebook.