Roddenberry just wasn't a prude about sex and sexuality--never was. He also came from the pulp sci-fi/comic book era in which wild ideas (by our current standards) wasn't quickly scoffed at on internet message boards.
I actually enjoyed Roddenberry's novelization of TMP and thought he really added a lot of depth to a story that otherwise relied mostly on visuals except towards the end. He even came up with a very simplified "layman's" explanation for warp drive that I think is the best in any Trek novel. The various in-universe background footnotes he added here and there also featured one that addressed (albeit indirectly) the Kirk-Spock slash fan fiction that had been going on in fanzines for quite awhile.
Free your mind, and the rest will follow.