I think several people have already expressed much of what I might say. But in the interests of clarity, my opposition to intermarriage is strictly concerning the marriage of Jews to non-Jews. What non-Jews do amongst themselves concerning the customs, ritual laws, and cultural practices of marriage is not my business, or the business of any Jew.
Intermarriage between Jews and non-Jews is significant for two reasons: one is that we are prohibited by our religion from marrying people who are not Jewish. The other is that when Jews do intermarry, the result is not only chaos because of the uncertainties introduced into our community vis-a-vis identity and affiliation, but also because the vast majority of intermarriages reflect assimilation of Jews into the secular culture, and intermarriage tends not to help this problem. The children of intermarriages are not Jewish if the Jewish spouse is the male; but even when the children are Jewish (the Jewish spouse being the female), intermarried families tend to either give children weak Jewish identities and vastly insufficient Jewish education and experience, or no Jewish identity at all.
In other words, it eats away at the fabric of Jewish existence. Some intermarriages actually lead to the formal apostasy of the Jewish spouse (such as the one apparently being protested in the OP article), which is not only a sin in Jewish religious teaching, but it is of course an even more violent ripping away at Jewish existence. But even the intermarriages that don't lead to apostasy overwhelmingly lead to secularist assimilation.
Judaism is a socioreligious ethnicity (ethnicity not in a racial sense but in a cultural sense): that is to say, it is an identity which incorporates elements of religion, of nationality, and of culture-- all inextricably linked with each other. Intermarriage not only transgresses the religion, it aids in destroying the culture as a whole, since Jewish identity requires not only blood belonging or adoption (i.e., conversion), but to be functional and meaningfully effective additionally requires education and experience of living a Jewish life. This nearly never occurs with intermarriages. The one or two examples of intermarried families I have encountered where it did occur, the non-Jewish spouse eventually converted to Judaism.
The requirement of endogamy has nothing to do with superiority or inferiority, nothing to do with wishing to exclude certain types or races or whatnot. It has to do with the Jewish People being preserved, with Jewish culture and religion continuing and with the Jewish family being a stabilizing and cohering force to keep the Jewish People and our tradition alive and effective.
If a non-Jew and a Jew wish to marry, Judaism provides a way to facilitate that happening: the non-Jew can convert to Judaism.
It's pretty to talk about love conquering all, but the truth is that everyone in any given society has responsibilities outside themselves and their own desires. No one should force a Jew to marry or not marry whoever they want using violence, abusive language, or public humiliation. But to be a Jew is to have responsibilities to the Jewish People, to our covenant with God, to our tradition, to our future. Those responsibilities can transcend even the romantic desires of our hearts. If a Jew chooses to marry a non-Jew, they must live with the knowledge of the wrong they are doing to the Jewish People and the Jewish tradition, and they must accept the consequences of their actions (their children not being Jews, their spouse being unable to participate in Jewish observance and ritual, etc.).
I don't say this lightly. Twice in my life, I was involved with a non-Jewish woman I loved and would have married. Both times, the woman refused to convert, and we ended the relationship. I accepted my heartbreak as the price of being faithful to the responsibilities I have to my people and my God; the women, being the excellent individuals they were, understood, respected, and supported my stance. It was hard, and so I know what I mean when I say this is what responsible Judaism can sometimes ask of us; but it would have been utterly selfish to do otherwise.
I can imagine that some of those who have expressed criticism (or intolerant vituperation) of the rejection of intermarriage do so because they believe in some sort of universalism, likely of the sort that makes light or condemns boundaries such as cultural or national divisions, or perhaps even religious divisions. Thus, any reason of identity affiliation that would prevent two people from being together is deemed offensive.
Personally, I think universalism of that sort is foolish. It devalues the uniqueness and specialness of our differences. I am much more interested in pluralism: wherein the different cultures, nationalities, ethnicities, etc. are valued for their singular and remarkable variations and individual natures, and each is tolerated.
If my desire and my dedication to preserve my people, my culture, and my religious tradition is unacceptable to you (whoever you are), so be it. I can live with that just fine. Feel free to call me whatever names you like because of it. But my people have struggled to persist through literally millennia of war, oppression, subjugation, attempts at genocide, and centuries of attempts to force us to assimilate or apostasize. We did it by sticking together, by valuing our culture, our religion, our tradition, and by reinforcing the responsibilities we have to one another over our own selfish desires. Like many of my colleagues, I work hard to counter with education and inspiration the soft threat of secular assimilation and spiritual apathy that is besetting our community; if part of that is having to get called stupid names by people for trying to keep my people going into the future, I assure you, I will not lose a moment of sleep over it.