What is the ruling on intentionally exposing seminal fluid for the purpose of lust without committing fornication?

Ejaculating semen through masturbation by hand, or looking at someone with whom enjoyment is forbidden, or intentionally thinking about a forbidden woman, and so on, is not permissible. However, ejaculating it through one's wife or slave-girl is permissible in every way, except for anal intercourse or vaginal intercourse during menstruation or postpartum bleeding.