Modified version of Hindu Yoni-Lingam symbols, as included on trump card 7 ("The Chariot") in the Rider-Waite-Smith tarot deck.  This could be used to explain the hypothesis that some uses of some versions of cross symbols are intended to have sexual symbolism (see en:Talk:Cross etc.) -- though it's highly unlikely that such sexual symbolism could be the "true" origin or meaning of all cross symbols in all cases.
Took Image:Rwt_major_07.png (raw scan of "The Chariot" Tarot card which is in public domain in the United States in this uncolorized form), did a vector trace of the part of the image containing the symbol, and subsequently heavily cleaned up and symmetrized the vector trace, also adding a simple uniform red background color to the central symbol.
Original author was Pamela Colman Smith in 1909 (modified from an abstract stylized pseudo-Egyptian lotus motif in an earlier version of the Tarot card by Eliphas Levi); subsequent image manipulation and clean-up by User:AnonMoos