@Rich
First of all, whataboutism isn't a viable argument for Christian morality. I mean, I can make up a morality ten times better than all those moralities combined and if I have to draw from an existing source of morality, I can chose from Buddhism, Confucianism, Shintoism, Taoism, etc. all of which are arguably better than any Abrahamic form of morality. You need to make an actual argument for Christianity instead of saying some other forms of morality are worse.
Second, Islam does legalize polygamy but it does not encourage it. If you were to actually read the goddamn Quran (which is something you need to do if you want to even talk about Islam), you would know that it recommends not to have multiple wives but that if you are going to, you need to follow a set of rules:
1. Possession of sufficient financial resources to provide for all expenses in the family
2. Physical prowess in for completely satisfying the sexual desires of each wife
3. Observance of complete justice and equality among each wife in every way without any favoritism
4. You may only have 4 wives
Third, given these rules you would be better off being a wife to a guy in the Caliphate than in the West. If a king or noble who was with another woman started a relationship with you in the West, you would not receive any support from your lover, your lover would have vested interest in making sure that no one would know of your relationship together perhaps even killing you if you attempted to get the word out, and if you had a child that child would be seen as a bastard and ostracized by the general population if it became known he was illegitimate.
@SolarCross
I know, lions, gorillas, bovines, etc etc. I am not really against polygamy myself, there is a eugenic dividend at least.
The fact that you're not against polygamy is really troubling. Furthermore I find the idea that there is a eugenic dividend in what women a polygamous guy marries is hilarious. Oh you should not look into the races of wives polygamous guys had in the Middle East, why you would be screaming bloody murder.
There is a social stability cost though because for every satisfied alpha with all the poontang there will be some frustrated incels who potentially could be provoked to revolutionary fervour.
Islam limits the amount of wives you can have (you can only have 4). Also I find it ironic that you are accusing revolutionary fervor to Islam while profession Christianity as the opposite. Islam encourages assimilation into local traditions and was more focused on reforming than dismantling the status quo. The reason why Islam allowed polygamy in the first place was because polygamy was widely practiced in 7th century Arabia and Mohammed decided that instead of banning it altogether, he would discourage it instead. Even alcohol is discouraged, not banned. The Quran says it is
preferable to not drink alcohol but not necessary.
Meanwhile Christianity advocated for the dismantlement of the Roman establishment, to forego it's state customs, to rebel against the status quo, and then to follow another authority figure instead.
or aggressive warmongering (like Islam)
It might seem to you like warmongering if you only read European history and somehow think it's the only history you need to know. Of course to Europe (or at least bad history textbooks with a big Eurocentric bias i.e. your favorite) the Caliphate seemed like a huge evil force threatening all modern European values despite such values never even existing in medieval times. But the Caliphate, and this might come as a shock to you, had actual objectives and those objectives weren't blind expansionism because that's a bad objective that will lead to no where. The original Arab conquests for example, weren't conquering territory for the sake of territory but were doing so because population density in Arabia was too high and migration was inevitable. Even if Islam never existed, the Arab migrations would've still happened. They need to settle in new places so they conquered. The conquest of North Africa was to take it's wealthy cities and to have a better hold on the Mediterranean. The conquest of Spain started out as a war to secure the border between Morocco and Spain but ended up as a fully-fledged conquest. The conquest of Malta and Sicily was, once again to have a better hold on the Mediterranean. The conquest of Central Asia was to get rid of those pesky tribes who frequently raided Umayyad cities. Saying that Islam is evil because it conquered a lot of things is like saying the British Empire or Christianity is evil because it conquered a lot of things.
In short you're being ridiculous.