How do same-sex partnerships actually hurt heterosexual marriage?

The whole premise of the U.S. Constitution, and the need to actually have a government is to ensure that one person's rights end where the next person's rights begin. When two consenting adults of the same sex choose to make a lifetime commitment to each other, what are they taking away from anyone else?

How will anyone's heterosexual marriage be stronger just by making sure that two people of the same sex can't get married?