One way that polyamory might not be a kick in the teeth to evolution-- if extended blood families are difficult to recruit for child-raising in the modern world (as seems to be the case), and it's to the advantage of both parents and children to have additional help with child-raising, then the ability to recruit additional adults (whether in sexual relationships or not) should be selected for.

And, of course, you can kick evolution in the teeth any time you feel like it, and I think people spend a lot of time doing just that. [1] It's just that you can't get away with it indefinitely.

[1] It generally doesn't seem to occur to people in dowry cultures to marry their daughters to people from non-dowry cultures, thus saving the money and still getting grandchildren.

I use this as evidence that memes can trump genes fairly easily.