Yes, they are suing for marriage...

... because it's right.

The Defense of Marriage Act (DOMA) is an attack on states' rights. If you believe that states should have the power granted them by the Constitution, you need to live with the fact that the rule must apply whether you happen to agree with that state or not.

Note that this suit (reported here by the Boston Globe) to overturn DOMA, if it succeeds, would definitly not force marriage equality on South Dakota... it would just allow Connecticut and Massachusetts laws to apply to Federal benefits.

Fifteen gay and lesbian residents from Massachusetts who wed after this state legalized same-sex marriages plan to file a discrimination suit today, challenging a federal law that defines marriage as a union between a man and a woman.

...

"It hurts," said Dean T. Hara, who was married to Studds from May 2004 until the retired congressman's death in October 2006, as he discussed the federal government's denial of a $255 lump-sum death payment and thousands of dollars in benefits as the surviving spouse of a retired federal employee. "But at the same time I realize that I, as a man, need to stand up for what I believe in. This is a nation of laws, and we're all supposed to have equal treatment under the law."

...

"We've got this major federal statute that inflicts really substantial harm on very large numbers of gay people just for being gay people," said Andrew Koppelman, a Northwestern University law professor. "The federal government declares to these people that it regards their marriages as worthless and would not give those marriages the protection and recognition that it gives to all other marriages.