An honest take is that the leftists are either atheists or non-Christians
that are also moral relativists - that is, Islam is no better or worse than Christianity and in fact preferable to “inflexible” anti abortion and Catholics Evangelicals and Orthodox Jews. Could be because of anti establishment sentiment, sticking it to whitey or “the man”...who cares, this sentiment should be repelled by any and all means with extreme lack of prejudice.
Interestingly “libertarian”s and libs alike conflate Constitutional freedom of religion with the establishment of a state religion (unlike properly educated pre-Millennial Conservatives who know that was because the English king was also the nominal head of the C of E) the paradox is using the Bill of Rights to ban all religions from the public square then make illegal concessions to an even more violent and intolerant Islam
left out pro-traditional marriage but you get the picture