>>1313041Disclaimer: this is a pointless argument to have. But here's something that has worked for me in the past...
1.) Show them things like this. They don't realize how different reality is from 2d anime figures. I am attracted to the girls on the left; the girls on the right register as children to me, as they should. Anyone with a working mind and eye should be able to see the difference here. One's a cartoon, one is a person.
I think if they can't understand this, then you should just stop and leave the topic be.