Why always the marriage penalty?

I have no idea if below is what will happen or not….but every proposal floated has had a marriage penalty in it.  Under the scheme below….it would be smarter economically for two high earners to live together (and not marry) since the tax hike would not hit their household until $800,000. If married, it hits them at $450,000.  See below



“….McConnell and Biden, who served in the Senate together for 23 years, are closing in on an agreement that would hike tax rates for families who earn more than $450,000, and individuals who make more than $400,000, according to sources familiar with talks…” (From Politico)