What rights and benefits do married people have

That's a good question. Here's a few;
1) In most states under most circumstances, prenups not withstanding, in the event of a divorce assets would be split 2 ways.
2) Spousal visitation rights in hospitals and prisons.
3) Next of Kin rights with regard to insurance, being told sensitive information by police/doctors etc.
4) Right to apply for certain documents. (It's going to be interesting when someone applys for a K1 "Marriage" Visa.)

The get biggest marriage 'penalty' is actually for widows/widowers. Widows/widowers get to keep the married tax allowances and not revert to the single allowances. So if a widower and widow marry, between them they lose half of their allowances.