Since Islam is at odds with the same western democracy that the liberals/left hate, it serves the same purpose; to bring down the government of the United States, weaken it, restrain it, etc. So they ally themselves and defend fascist Islam because it suits their purpose.
Leftists don't exhibit any long-term thinking...that doesn't figure in...so the idea that by enabling fascist Islam that someday all their queer brethren, all their lesbian friends, all their degenerate, excessive behavior will be brutally surpressed when a fascist Islamic state takes over the United States and subjects them to Sharia law isn't even a thought to them. It never crosses their mind. They don't think that far ahead.