All Praise to Allah Subhanahu wa Taala and blessings of Him be on Prophet Muhammad peace be upon him His family and companions
Islam does not allow sex without marriage.
Narrated `Uqba bin Amir: Allah's Messenger (ﷺ) said, "From among all
the conditions which you have to fulfill, the conditions which make it
legal for you to have sexual relations (i.e. the marriage contract)
have the greatest right to be fulfilled."(Bukhari)
You said you have been rejected several marriage proposals but you could find In Sha Allah someone if you try to find someone with the same issues as it would be hard for the other person too to find someone perfectly fit.May Allah Swt make it easier for you.
Three groups of people Allah obliged Himself to help them: Mujahid in the cause of Allah, a worker to pay his debt, and the one who wants to marry to live a chaste life." (Tirmidhi)
There is no niyat in salat which would make your sex without marriage permissible kindly ask the mufti regarding this statement.
Temporary marriage or Mutah was abrogated by Prophet Muhammad peace be upon him and it is not allowed in Islam now (Sunni view),
Narrated `Ali bin Abi Talib (Ra):
On the day of Khaibar, Allah's Messenger (ﷺ) forbade the Mut'a (i.e. temporary marriage) and the eating of donkey-meat.(Bukhari)
Rabi' b. Sabra reported on the authority of his father that Allah's Messenger (ﷺ) forbade on the Day of Victory to contract temporary marriage with women.(Muslim)
It was narrated from Al-Hasan and 'Abdullah, the sons of Muhammad, from their father, that 'Ali heard that a man did not see anything wrong with Mut'ah (temporary marriage). He said:
"You are confused, the Messenger of Allah forbade it, and the meat of domestic donkeys on the day of Khaibar."(Nasai)
It was narrated that Ibn 'Umar said:
"When 'Umar bin Khattab (Ra) was appointed caliph, he addressed the people and said: 'The Messenger of Allah permitted temporary marriage for us three times, then he forbade it. By Allah, If I hear of any married person entering a temporary marriage, I will stone him to death, unless he can bring me four witnesses who will testify that the Messenger of Allah, allowed it after he forbade it'."(Ibn Majah)
Also read this about Mutah
Sex without marriage is Zina is Islam,
And do not approach adultery – it is indeed a shameful deed; and a
very evil way.(Quran 17:32)
Allah Swt and His Messenger knows best.