Adult suckling of females other than their Spouse is not a sin
Whilst not practiced widely, adult suckling is permitted in Islam thus not a sin.
see the related hadiths;
https://sunnah.com/muslim/17 [an authorized Islamic site]
The general rule is, it is sinful for an adult male to accompany an unmarried female BUT if the unmarried female allowed the adult male to suck her breasts, then he can be in private company with the female.
I believe [need to find evidence for this] staunch Muslims will obey the above rules re adult suckling since it is not a sin according to God.
Adult suckling (Arabic: رَضَاعَةُ الْكَبِيرِ), or the act of breastfeeding a male adult, is mentioned in several relied-upon collections of hadiths. According to five hadiths in Sahih Muslim, Muhammad once plainly instructed the daughter (or wife -- sources are unclear) of a companion named Suhail to suckle a "grown-up" freedman named Salim so that Salim would become the daughter's mahram, or a relation whom the daughter could no longer marry, and thus render Salim's cohabitation with the family appropriate and legal.
Following the publication of a book in Egypt promoting adult suckling by Dr. Abd Al-Mahdi Abd Al-Qadir, a scholar at al-Azhar (the "Harvard of Islam"), another scholar at al-Azhar, Dr. Izzat Atiyya, published a fatwa promoting the practice in a widely-read weekly magazine in 2007, causing national and international controversy.
https://wikiislam.net/wiki/Adult_Suckling
Of course most modern Muslims will not practice adult suckling with strangers and women who are not their wives.
But they are merely following normal secular morality and ignoring what is permitted in their religion.
However they cannot condemn nor punish those who practice such adult suckling otherwise they will be condemning their God's ruling.