Keiko's clearly capable of consent though her form seemed distinctly pre-puberty so I'm not sure how it would have worked.
Hugging and having adult conversations certainly is not wrong but I think puberty is a cutoff for the sexual situation. If it were a permanent situation, and Keiko was through puberty, I think O'Brien would have adjusted to it. ...
This echoes my thoughts on the matter. It would be awkward at first, but she's still an adult, still his wife, and still the same, intellectually and emotionally. He could adjust.