Is Zina Against Fitrah?
I think sexual desire is in the fitrah and so is having a proper family system. A child requires a family for proper growth. It’s a matter of prioritising. Moreover we cannot fulfil all the desires of the fitrah in this world. Generally sexual relations outside of marriage are considered wrong or fawahisha by everyone. Islam prohibits such relations before marriage as a principle of Sade zariyah.
Sex outside of marriage(cheating on your partner) is considered wrong everywhere.
If Fitrah means human natural instinct, I agree with Zhoaib’s question. If there was no guidance from any religion, human nature will not tell a person to get married or if engaging with multiple partners is wrong. So, perhaps then it is not correct to say that Islam is based on Fitrah. Although once someone finds out that Allah has prohibited such acts, then the person will be obligated to follow.