The forehead ribbon means "to regulate oneself," and as such, very few are permitted by the owner to touch it, unless they are either family or cultivation partners (aka lovers).
"According to the founder of the GusuLan Sect, Lan An, had said that one can only let go of all regulations when they are with the one they love and cherish. So, the message that has been passed down the generations is that, Lan sect’s forehead ribbon is a special object that is very, very personal and sensitive. One cannot easily give another permission to touch it, one cannot take it off as they please, and one absolutely cannot tie it on someone else. It is prohibited. That is, unless, unless…” - Lan SiZhui trying to explain it to Wei Wuxian 😂 in Novel Chapter 45
In the donghua though, Lan Wangji explains this in a basic way during the Tortoise of Carnage slaughter, when Wei Wuxian uses it to tie up his leg.