All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, we supplicate Allah, The Exalted, to safeguard you from every misfortune, to help you obey Him and ward off the wiles of Satan from you.
Dear brother! You should not surrender to these whispers and insinuations, since such things only arouse one sexually and lead him to commit immorality (such as adultery, fornication, etc., …) and what incurs Allah's Wrath.
Here, you should remember the saying of the Prophet in a sound Hadeeth reported by al-Bukhari and Muslim: "Allah has written for the child of Aadam his/her share of Zina (fornication and adultery) which he/ she commits inevitably. The Zina of the eye is the sight (i.e. to gaze at what is forbidden), the Zina of the ears is the hearing (i.e. listening to what is forbidden), the Zina of the tongue is the speech (i.e. saying what is forbidden), the Zina of the hand is the touching (i.e. illicit touching), the Zina of the feet is the walking (i.e. to forbidden places) and the heart desires and wishes, while the private part ratifies that or denies it.”
You should be keen on doing what benefits you in this worldly life as well as in the Hereafter. You should think of what pleases Allah and know that Allah, The Great and Almighty, Says (what means): {...Indeed, the hearing, the sight and the heart - about all those [one] will be questioned.} [Quran 17: 36]
There is no doubt that thinking of women and having sex with them constitutes a motive to commit Zina (adultery and fornication). So, a Muslim should distract himself from such thinking and not indulge in it.
As for mere desires for sex without further action, it is pardoned, Allah Willing, for the Hadeeth in which the Prophet said: "Allah has forgiven my nation for the whispers that they have except if they speak about it or act according to it.” [Al-Bukhari and Muslim]
As regards Ghusl, please refer to Fataawa 81699 and 82245.
Allah knows best.