It gives massive structural advantages to couples with low income, in the form of a lower marginal tax rate. Does it really discriminate between single and dual income though? I wasn't aware of that.
Let's say you made 150k and partner made zero. If you were filing as single, high tax bracket you would get is 24% at everything past 100k. Married filing jointly, 22% at 94k. Also, you could borrow your spouse standard deduction as well to help reduce the tax requirements.