@Nonna_SoF:
I can't find any data on that (every source just points to the same shitty study on 18 het couples) but I can imagine it to be low enough to vanish in the metabolic noise. (Chewing and speaking are essentially free, so other tongue-intensive activities might be as well.) This assumes no other action and a relaxed position, in which case I even suspect the facesitter to burn more calories than the facesittee.
Anyway, my inability to read the room in combination with utter lack of self-control made me run the numbers on how "realistic" this actually is:
Good news: due to the contained electrolytes, you don't have to worry about water toxicity. Probably. My degree isn't in medicine.
Bad news: I don't know how much lubrication in ml can be expected to be produced or consumed, but let's assume a "safe maximum" of 200 ml which as about as much plasma as there is in western blood donations.
This means that in the absolute best case scenario, you have to eat out 29 women to exhaustion in one day with minimal losses. As plasma may take up to 48 h to regenerate, sustainability requires thrice that amount on rotation.
You'd still get malnutrition though.