Spencer Sex Tape Better than Girl-On-Girl

The Heidi Montag sex tape saga has exposed a glaring hypocrisy in the world of celebrity sex tapes -- straight sex trumps beautiful, innocent, glorious girl-on-girl action.

Sources close to the Heidi sex tape deal tell TMZ ... Vivid Entertainment is only slightly interested in the rumored video with Montag and the Shannon Playmates. They are, we're told, way more interested in the Speidi videos.

We're told Vivid honcho Steve Hirsch is very clear ... the money tape is the one featuring Heidi and Spencer Pratt -- a tape he believes could surpass Kim Kardashian's sex tape in sales.

To sum it up ... video of Spencer Pratt is somehow more appealing than video of naked Playboy Playmate twin sisters. What is this world coming to?!?!?!?

UPDATE: Kristina Shannon says she is "100% positive" that she is not in any sort of sex tape  with either her twin sister Karissa Shannon or Heidi Montag.

Kristina adds, "I'm saddened by the situation with Heidi and my sister."