Suggesting that being okay with discrimination, as a broad statement, is "anti-american" is ridiculous. The government discriminate based on sex in cases of alcohol, cigarettes, pornography, language, voting, and more. The government discriminates based on race in regards to things like affirmative action. The government discriminates with regards to sex on issues like females on the front lines. To suggest discrimination, blanketly, is anti-american is ridiculous.
The issue of potential "Unamerican" feelings towards discrimination is when one pushes for disrimination that is unconstitutional. Specifically, discrimination that wrongfully removes someones rights due to their sex, skin color, age, religion, etc.
In the case of marriage, sex is being discriminated against and sex holds a moderately high threshold of requirements on the part of the government to justify discrimination based on it. A threshold that I do not believe the discrimination inherent in marriage reaches.
"Number of people" is not a protected status under the Equal Protection clause. There's nothing unconstitutional in suggesting that something two people can engage in must constitutionally be allowed for three people to engage in, or four, etc. Even if it was a protected status, it would rank at the lowest tier which does not require much of a threshold to justify government discrimination against them. And considering there are significantly more issues surrounding polygamy than there is with same sex marriage as to why its in the governments interest to discriminate against it, its reasonable to suggest that needing to meet a lesser threshold with far more ammunition is likely to happen. With things such as the potential tax loophole that opens up in allowing large amounts of people to enter into a binding agreement that would allow dozens upon dozens to file taxes as a single entity requiring significant rewriting of tax laws, or issues concerning incidents where a person is married to two separate people but those people aren't married to each other creating significant court issues regarding rights to act in the middle persons steed, the issues present before the government in legalizing polygamy are far larger than there are regarding legalizing same sex marriage.
So sure....not allowing polygamists to marry multiple people is discrimination. Perfectly legally and perfectly "american" discrimination that is wholly different than the sexual discrimination present with marriage.