I'n not sure if I understand you correctly:
Do you mean: because Bettina realized that Marc's feelings for Kay were deeper than just physical, jealousy would not be appropriate?
In her case, I would be even more jealous.
And: before Bettina began to attack Marc in the shower, she said to him something like: so tell me what I must do, do you like it from behind? (while standing behind him and touching him). And right after that she said: I can't even be jealous.
It shows a frustration. She can't do it like Kay (the way Marc likes it) simply because she is a woman. She can't compete.
No quite opposite. When we are shown Marc and the stranger from the club not working out in the stall, we know then that Marc's connection to Kay exceeds physical pleasure. If Marc just wanted gay sex, he surely could get that easily with those eyes and good looks of his - nor would not have rushed out on his pickup - what 3 mins and it''s done... ? That's not what Marc wanted or needed.
I agree - this is the overwhelming reason Bettina should be jealous but she does not know that yet. If Marc needed just physical, then she and Marc maybe still had the emotional connection they needed for their partnership to possibly survive but Marc, and we are shown this, realizes that his feelings for Kay exceed those of Bettina, emotionally.
In the shower, Bettina's comments provide insight. Marc's attraction to Kay is NOT just a physical pleasure of a penis vs. vagina... and it is that which drives her "like it from behind..and I cannot even be properly jealous".. I do not think Bettina realizes it is emotional now with Marc and Kay - not just physical.. Did that make sense, I'm not sure I said it well.. V.