According to soccer's Laws of the Game, Law 04, Section 5,
“Equipment must not have any political, religious or personal slogans, statements or images” and “slogans, statements or images related to the following are not permitted...any organisation who aims / actions are likely to offend a notable number of people.”
Responses to use of the LGBT rainbow in soccer show that this is indeed "likely to offend." The LGBT rainbow on equipment is in violation of Law 04.5.
For example, consider the case of U.S. player Jaelene Hinkle, a Christian called up to the U.S. Women’s National Team (USWNT) in June 2017. She declined to play because she, as a Christian, felt she should not wear the jersey with the LGBT rainbow.
In 2018, Croatian player Josip Brekalo declined to wear a rainbow captain’s armband when his Bundesliga team, VfL Wolfsburg, initiated a program to “support LGBT communities.” Brekalo explained, “I was raised very religiously, so if someone chooses a different way of life, that’s okay with me because that’s their thing, but I do not have to and I do not want to wear a special symbol for someone else, which opposed my Christian worldview.”
In 2016, during the Premier League’s “Rainbow Laces” campaign, in which teams made LGBT rainbow laces available for players to wear on their cleats, “a large proportion of the negative comments” about this came from “the Muslim world.”
From late November to early December 2018, the Islamic Republic of Iran blocked broadcasts of Premier League games during the Premier League’s “Rainbow Laces” campaign.The government of the Islamic Republic of Iran considered this offensive to its Islamic values.
The 2018 Premier League’s “Rainbow Laces” campaign included LGBT rainbow laces for players’ cleats, LGBT rainbow captains’ armbands, an LGBT rainbow
corner flag on the pitch, and the LGBT rainbow on substitution boards.
This blog post is from pages 6-7 of the 2019 report, "Let All Play: Yes to Soccer, No to Politics". The footnotes in this blog post are on pages 26-27 of the "Let All Play" report. CLICK to view the full report.