You know there is nothing stopping Morrigian seducing Alistair or Loghain without the protagonist giving her permission. Honestly Bioware have retconned bigger things. Loghain in particular strikes me as someone who would enjoy it.
That can't happen because:
1) Alistair doesn't trust Morrigan at all. She even says it to you if you ask why don't she go to the him and she says he doesn't trust her.
2) They can't retcon it because for an OGB to exist, the Wardens must all be alive. If a Warden dies and theres an OGB, then how they hell did Warden die from killing the Archdemon?
3) Loghain would never enjoy laying with a witch. In fact, he seems pretty suicidal after becoming a Warden so he'd probably just tell Morrigan to get lost if she asked.