I thought that both Brenna and Yuta were age-appropriate for Riker. So I don't get the "ick" factor. (It's hard to believe that a grown woman would even seriously use the word "ick".) Even though Yuta was actually older, she at least looked to be about the right age for Riker. Creepy people are also easily creeped out, I suppose.
Also, I always got the impression that Yuta was brainwashed in such a way that killing Lornack clan members was like a drug, like crack for her. She hated her addiction but was addicted nonetheless and had no choice. That's why she was intent on touching Chorgan. Since he was the last of the Lornacks, her addiction would be over or Riker would kill her. Either way, she would finally be at peace.