What was the context on why that subject was brought up? Where they hanging out one day and Supes just mentions it to which Kara responds, "Uh....okay?" (which would suggest that Clark had some socially taboo thoughts on his mind). Or was Kara seriously considering wedding her cousin?
Either way, such an awkward conversation.
I believe the context was Supergirl wanting to find Superman an appropriate wife. He then mentions she would be perfect but Kryptonian legalities forbid it. Later Supergirl finds a woman who is her double, and Superman hits it off with her.
Said woman was super on her world, but not on earth, or superman wasn't super on her world.. some such crap to keep them apart.
It was a very creepy story for the silver age.