It's not just disabled people or people on benefits that this discriminates against.
Since his father's death, my husband runs his family's crofting business. The house we live in has been paid off for years, no one here claims any benefits, and we work hard. But we'll never make that kind of money, living out here. Nor will most other people living in rural areas, or virtually anywhere outside the southeast/London area.
And when you start looking at other groups who statistically earn lower incomes (i.e. women and minorities), it becomes very clear that the only people the government believes should have the right to marry whom they please are white, middle-class, males from the home counties. As usual, they care nothing for anyone who falls outside that category.