Either the standard applies or it does not.
Either personal morality and fidelity to one's spouse matters or it does not. If you argue that someone's private life is not relevant to his ability to serve, you cannot suddenly change your mind and hold someone from the other political party to the a higher standard. Either the standard exists or it does not, and if it exists it applies universally. The standard does not change based on partisan convenience.
In the 1990's and into the 2000's, conservatives spoke out often about how personal moral character was a qualification for public office. That started to change in 2015, and many conservatives outright abandoned that standard when Donald Trump became the Republican Party nominee for President. Some even made self-righteous arguments about "forgiveness" or made ludicrous comparisons to King David in order to justify supporting Trump.
Look, I get it. Elections are not a simple black and white choice, and you have to take the entire political landscape into account when deciding how to vote - and that includes the damage the other candidate would do if elected. I voted for Darrell Castle in 2016, but if you are one of the 63 million people who voted for Donald Trump, fine. I will be one of the people voting for Trump in 2020.
But the one thing you may not do is excuse Trump while condemning Ilhan Omar for allegedly having an affair with a married man - breaking up his marriage and betraying her spouse. If you declare that "I did not vote for Trump's personal life," then you do not get to use Omar's alleged behavior as a disqualification either.
This was such an easy choice, and I do not know why conservatives did not choose the more nuanced option. I do not understand why it has to be all or nothing for some Trump supporters - if we support Trump we must effusively praise him and never criticize him. It is intellectually lazy.
The principled answer to Trump's immorality is not to abandon principles we have held for decades, but to say this: "I condemn Trump's immoral behavior and it does make me question whether he can be trusted. However, I will vote for him anyway because..." And then you explain why you will be voting for him in spite of his personal morality - solid conservative policy, fighting the Left, and so forth.
Not only does that preserve your personal integrity, it leaves you free to criticize opposing politicians for sexually immoral behavior without justified accusations of hypocrisy. I can criticize a Democrat's sexual sins because I openly criticized Trump for the same thing. Someone who excused Trump does not have that option, at least not with any moral and intellectual consistency.