Seattle (CNN) - David Wing-Kovarik and his partner, Conrad, were ready to adopt a child.

They moved through all their requirements smoothly, even completing an orientation and training course for prospective parents.

Then they were confronted with their first real stumbling block.

"Our adoption agent said, 'Well, you both look the same on paper, so who's going to be the parent?'" Wing-Kovarik recalls.

In Arizona, where the couple lived at the time, only individuals and legally married couples may adopt from the U.S. foster care system. But because a same-sex couple cannot legally marry in the state, only one parent can be granted legal rights to the child.

"We saw (it) as a disadvantage to the child," said Wing-Kovarik, 47. "We, frankly, got very angry about it when we thought about everybody else that was in the (training) class. None of them were asked this question. And it came down to the fact that we were a male couple. This was when we first experienced how being that gay couple just adds to the complexity of the whole process. It makes it much harder."

In 18 states and the District of Columbia, same-sex couples can jointly petition to adopt a child. But in the other states, such as Arizona, the law either restricts joint adoption or is unclear.

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