I think that Anna has chosen for a risky way to show Ann that she (Ann) is not a suitable bedpartner for Alex and vice-versa. She has maybe invited her friend and forced Ann and Alex into one bed with the result that nothing sexual has happened although the relation Alex-Ann was very 'friendly' yesterday-evening. Anna might be 45-50 kg, Ann a 15-20 kgs more; even if Ann would like the rough sex of Alex, what I don'believe, then it would be physically impossible. Anna accepts a lot from Alex, who is like most guys in VV, a lousy lover. Even after an hour of sex she tends to masturbation when he is gone. They all have seen too much porn.
Anna's girlfriend seems to know beforehand when she needs to be invisible; I keep thinking about a secret room in Tula. After a party or visit the girls are brought home with male company, but this girl disappears alone at about 7.00 a.m.