Allowing the British singer and songwriter to perform at the Mawazine World Rhythms festival in the capital Rabat would tarnish the image of the north African kingdom, say powerful opposition Islamists.
The writer of "Candle in the Wind '97"', the best-selling single of all time, according to the Guinness Book of Records, has championed sexual freedom and campaigned against the spread of AIDS during a four-decade musical career.
"Elton John is one of the best artists in the world. He is great and extraordinary when he appears on stage. That's why we invite him and welcome him to the Mawazine festival," festival director Aziz Daki told Reuters.
"The private life of a singer is not our business. We do not invite singers and artists after assessing their private lives."
The festival, backed by Morocco's King Mohammed, brings together musicians from 50 countries and has drawn criticism from Islamists who say such events encourage promiscuity and alcohol consumption, corrupting Islamic values.
"We asked the government to exclude this person from the list of artists invited to this festival. This man -- sorry, I should say this person, not this man -- is known for bragging about his homosexuality," said Mustapha Ramid, a leading parliamentarian from the opposition Islamist PJD party.
"Morocco is an Islamic state where stages should not used to allow a person with such a degree of debauchery to perform because we have to shield the young from such influences," said Ramid.
Khalil Hachemi Idrissi, editor of the French-language daily Aujourd'hui Le Maroc, said: "Such a point of view, if we do not pay enough attention, will quickly turn Morocco into an ostracised country among civilized nations."
Liberal intellectuals see the calls to scrap Elton John from the festival line-up as part of an attempt by religious conservatives to reverse the modernization of Morocco.
Daki said attendance at the festival would be the "ultimate answer" to critics. Mawazine runs from May 21 to 29 and is due to feature over 100 musicians including Britain's Mika, Spanish crooner Julio Iglesias and Lebanese star Elissa.