You taking lust out of the context of the passage: Epithumeo just means strong desire which depending on the context is good desire or forbidden desire (coveting what belongs to another). Epithumeo is a verb, an ACTION. The subject being addressed here is adultery (not Pornia). From this context Jesus is using Epithumeo as in the Septuagint 10th commandment not to covet (Epithumeo) your neighbour’s wife or as Jesus spoke Hebrew, the Hebrew typically translated ‘covet’ in the 10 commandments is Chamad.
The late Dwight Pryor a godly humble expert in the Hebrew and Greek comments briefly on Matthew 5:27-28: '...doesn't mean just seeing a woman and being attracted to her. That's not the concept here, the concept is where you ACTUALLY TAKE STEPS to try and bring about some kind of improper conduct or improper act.' [Paul, The Law And The Church/ Dr Dwight a Pryor/ Disc 1 - Track 6 - 0 minutes 58 seconds/
www.cfi.org.uk] So attraction or fantasy of marriage isn't a sin, it's the actual intention of pursuing an adulterous sexual relationship.
Chamad is‘... “to desire (brings damage upon the thing or person desired) and to TRY TO OBTAIN”...with the result that a legal maxim has become a rule for governing conduct...’ (Loader, William. The Septuagint, Sexuality, and the New Testament: Case Studies on the Impact of the LXX in Philo and the New Testament (Grand Rapids: William B Eerdmans Publishing Company, 2004) p 10 footnotes 7)
‘...the choice is indicative of THE FOCUS, which, while including passion, IS ON THE ACQUISITION of property.’ (Loader, William. The Septuagint, Sexuality, and the New Testament: Case Studies on the Impact of the LXX in Philo and the New Testament (Grand Rapids: William B Eerdmans Publishing Company, 2004) p 10)
‘Within the context of the tenth commandment the focus is upon possession of what belongs to another’s household, including a wife. The focus is therefore on the desire to possess, which includes house field, slaves and animals. Attention is being given not primarily to theft not to the act of adultery, but to the RESPONSE which might lead to both as forms of theft from one’s neighbour.’ (Loader, William. The Septuagint, Sexuality, and the New Testament: Case Studies on the Impact of the LXX in Philo and the New Testament (Grand Rapids: William B Eerdmans Publishing Company, 2004) p 11)
‘The verb [action] epituméo corresponds to a Hebrew word which implies a temptation to acquire ownership...which lends itself to the notion of acquisition of property.’ (Loader, William. The Septuagint, Sexuality, and the New Testament: Case Studies on the Impact of the LXX in Philo and the New Testament (Grand Rapids: William B Eerdmans Publishing Company, 2004) p 10 notes)
Chamand is a ‘...emotion of desiring included the act off taking possession...an emotion AND a corresponding ACTION’ [Brevard S. Childs, The Book of Exodus(Westminster John Knox Press, Louisvill, Kentucky, 2004) p426]
We shouldn’t take Matthew 5:28 out of its context it’s about adultery not Pornia; read from Matthew 5:27 all the way up to verse 32. It basically means any intention to cheat on your spouse, that intention in itself is adultery. If looking causes you to intend to cheat on your spouse, don’t look! If touching (‘friendy’ hugs, etc) causes you to intend to cheat on your spouse, don’t touch! Don’t think you can get around this by divorcing you spouse in order marry your new interest, because such intent is also adultery!
Such active intent of heart will lead to actual physical adultery. Nothing to do with a single person.
For as he thinks within himself, so he is...’ (Prov23:5).
‘Let the words of my mouth and the meditation of my heart, Be acceptable in Your sight...’ (Ps19:14).
If a married person commits sexual acts with someone other than their spouse it’s sin, same with their thought life.
If a person engages in sexual relations with someone of the same-sex it’s a sin, same with their thought life.
If a single person has sex outside of marriage it’s a sin, so thinking about having uncommitted non-marital sex is sin.
'Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure (hagnos), whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think (logizomai) on these things.' (Philippians 4:8)
As I showed in my #14 post marital sex is a holy act. If a married person fantasises about sexual relations with their spouse it isn’t a sin.
If a single person gets married to someone of the opposite sex to have sex they haven’t sinned. Therefore a single person is okay to fantasise IN PRIVATE about committed marital sexual relations with an imperfect imaginary/potential unmarried person of the opposite sex. Only fantasise about is allowed in real life. Staying away from sin in thought and deed. A single person can choose to either masturbate with such fantasy or just focus on the physical sensations.