Parashas Acharei Mos · Passuk 18:29
Chabad.org — English translation + Rashi
Torah Temima points to Yevamos 54b.
English: From where is ha’ara’ah (partial contact) derived for all arayot? They are compared to niddah: just as niddah incurs liability for ha’ara’ah, so do all the arayot.
Ha’ara’ah for all arayot
Partial sexual contact is sufficient for liability across the entire list of forbidden relations.