That's like pointing out the existence of veganism in an article on pulled pork recipes, or the existence of other religious faiths in an article about the mean

That's like pointing out the existence of veganism in an article on pulled pork recipes, or the existence of other religious faiths in an article about the meaning of Ramadan in Islam. The article is written as a response to a question about romance in long-term monogamous marriages, so of course it sees monogamous marriages as a given. Did you expect her to add a paragraph like "Hey, maybe if you can't find romance in a monogamous marriage you're just not made to be monogamous or married and that's fine too"? Do you expect articles on programming best practices to contain similar paragraphs ("Hey, maybe if you don't like fizzbuzz you're not really into programming and should try hardware instead.")?