yeah, i have to agree that this is not a topic explicitly discussed in the bible, just as pornography is never discussed in the bible.
however, i'd humbly suggest that the greek word "porneia" references all forms of sexual impurity, and it's literal translation speaks to the act of surrendering (even the concept of "selling off") one's sexual purity. it covers all forms of sexual impurity and fornication outside of marriage (in contrast with moixia, which is marital unfaithfulness).
it's also a reference of idolatry, which i think is quite an accurate label for any habitual sin.
i love that it uses the analogy of "surrender" or "sell" because our spiritual purity is a thing of value that becomes cheapened/lost when we don't hold it in esteem.
in the past, i've heard people say masturbation is okay when there is an absence of lust, and i can't really argue too much with that on specific verse. so, if one can masturbate without any lusting, then perhaps that might create a small bit of ground one can stand on. however, i would be very concerned with the slippery slope of that, along with the directive of making no provision for sin.
however, for me personally, i believe all acts of sexuality were exclusively created for the marriage bed alone.