Human psychology doesn't have a standard.

Monogamy is just one of a wide range of social arrangements that have existed all throughout human history, and continue to exist.

see: distribution of monogamy

Personally, I've been in multiple partner relationships. Are they perfect? No, absolutely not. But neither were some of the monogamous relationships I've been in.

I think the comic is a little off too, and I agree with you about how it portrays cheating, but it doesn't make sense to shit on all forms of relationship that you don't agree with.