What is the ruling on girls kissing each other on the mouth or elsewhere, and how does one respond to those who cite the hadith, "The Messenger of Allah, peace and blessings be upon him, would bite Zayd ibn Harithah's tongue," to permit that?

Kissing girls on the mouth is forbidden because it is a means to corruption. It falls under the principle of sadd al-dharā’i‘ (blocking the means to evil), which is established in Islamic Sharia, and it entails preventing anything that could lead to committing a forbidden act. Kissing on the mouth is a manifestation of desire (shahwa). For this reason, scholars have prohibited kissing mahārim (unmarriageable kin) on the mouth, and they have also prohibited a man kissing another man or a woman kissing another woman if done with desire. Even if it is not done with desire, the norm is for it to be on the forehead or the head, not on the mouth. It has been narrated from the Prophet, peace be upon him: "A man should not look at the ‘awrah (private parts) of another man, and a woman should not look at the ‘awrah of another woman. A man should not lie with another man under one garment, and a woman should not lie with another woman under one garment." As for the hadith about the Prophet, peace be upon him, biting the tongue of Zayd ibn Harithah, it is not authentically established.