Idrissa Gueye traveled south to Montpellier with his Paris St. Germain teammates for the team’s league game on Saturday but wound up watching it from the stands. Gueye was not injured, his coach said after P.S.G.’s victory: He “had to leave the team for personal reasons.”

Those personal reasons, a team source confirmed Monday, were that Gueye had refused to join his teammates in wearing a special jersey featuring rainbow-colored numbers created to highlight discrimination before Tuesday’s global events marking the International Day Against Homophobia, Biphobia and Transphobia. The jersey were part of a coordinated leaguewide effort.

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Gueye has not commented on his absence, but French news media reports, which were first to report the reason for Gueye’s missing the game on Saturday, quickly noted that he had also missed the same awareness-raising fixture last season. Last season’s absence was attributed to a stomach issue, but this year P.S.G.’s coach, Mauricio Pochettino, said Gueye did not have any health issues.

Activist groups were quick to denounce Gueye’s decision to decline to wear the jersey. Rouge Direct, a group that fights homophobia in sports, called on the French league, P.S.G. and Gueye to “explain themselves” and suggested Gueye should be punished for his actions.

For P.S.G. the issue of support for gay rights and ant-homophobia efforts is a particularly sensitive one. The team is owned by the Qatari state through its sovereign wealth fund, and Qatar itself has come under scrutiny amid concerns from the gay community about their safety when the World Cup takes place in the Gulf country later this year. Homosexuality is against the law in Qatar, as it is in other countries in the Gulf, but World Cup organizers have insisted that all fans would be welcome during the tournament.

Same-sex acts are also illegal in Gueye’s native Senegal, where ultraconservative groups have burned rainbow flags during public protests against homosexuality.

P.S.G. has not commented on Gueye beyond the comments made by Pochettino after Sunday’s game.

The club’s senior officials are currently in Qatar, according to a team representative, and it is unclear what measures, if any, will be taken against Gueye, whose contract with P.S.G. expires at the end of next month.

P.S.G. is by far the biggest team in France and attracts outsize interest, at home and abroad, because of its star-studded roster, which this year produced the club’s eighth French championship in the past 10 years. On Saturday at Montpellier, two of its star forwards — Kylian Mbappé and Lionel Messi — both scored goals, helping to highlight the specially commissioned jersey they wore on social media and beyond.

According to the team source with knowledge of P.S.G.’s internal conversations over the weekend, club officials had told Gueye that the team was committed to the campaign and that he had no option but wear the same jersey as the rest of his teammates if he wanted to play. When Gueye decided he would not, he was sent to the stands for the game, according to a person with knowledge with the incident.