While there are numerous things that we can call romantic or not, you are missing the most essential reason why marriage still matters — the more than 1,000 perks and protections the government bestows on people with a marriage license. (just ask same-sex people why they fought so long and hard for the right to wed; it wasn’t for romance!)
So, if people wish to marry “for ‘tax reasons’ For stability” why not? People want numerous things from marriage, and yes, especially for same-sex couples, social acceptability matters (As the co-author of a book on individualizing marriage, I can show you the research). You are coming from a privileged, white, heterosexual viewpoint, and not everyone is privileged, white or heterosexual.
Whether the government should favor people on their romantic/sexual life is another matter entirely; there’s a lot of good research on that as well.