I`ll take a guess and say that it is also there to emphasise the `taboo` or `forbidden` nature of some titles.. The idea that some acts are so shocking that the perpetrators are hiding their identity.
We all know it`s just an act, but as with most entertainments, we`re willing to pretend it`s real.
Happens a lot in rough/abuse, incest, older/younger, especially if it`s implied that there are much younger participants (eg loli & shota)
Who knows, that`s just my guess