It is not Spanish but English (although it may have a German origin). It would be another creative classification for sexual inclinations, this time for a gay, homosexual male attracted to other males. Etymologically it is related to Mars, which in English is Mars, and I do not know how it will end up being incorporated into Spanish, since "martico" sounds like Marcelo's diminutive, and looks a lot like marico. See Uranism.
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