Exactly, we're on the same page then. I don't see Cass being dismissive towards Jack, it's just that when you mention "relieve his frustrations" it made me imagine the worst scenario, like Cass waking up cuddled in another man's arms. Even if it happened just once (as part of the progress with Danton, for example) it would hurt Jack and I'd want him to make her know that, and set it as a boundary. May sound boring to set strict boundaries, but I feel extremely jealous for Jack of her emotions and want him to claim her love 100%, without a hint of polyamory.
I do love your idea that Lux may be more of an accomplice of Jack's perversion than just another taunting spectator. Lux could be a counterpoint to Cass' suitors, taking personal interest in promoting Cass and Jack's marriage, even while teasing him to death. Not a lover, but a friend (obviously, a very close friend, with benefits).
ForeverLosing hinted at Jack's progression a while back (or maybe it's my wishful thinking playing tricks on me)

I hope one possible ending will have Jack telling the goddess to suck it up. She'd have to bless his seed if she wanted her "god-children" to walk the earth. Have the goddess pump her blessing up Jack's ass, as he pumps his seed into Cass' womb.
Sorry, I know, too ridiculous for an ending - but might make for good comic relief.