For me I feel Anna does sexual things to help out a person she considers a friend or as a reward or pity.
I don't think the character considers anything they do to be cheating.
I've seen this attitude approach used in a few Avn's.
Bearing in mind that she believes David is a cheater for receiving a bj that never happened.
It's quite a hypercritical approach from Anna.
It happens. Quick story of my friend that dodged a bullet with a woman (girl, to be honest).
After HS he is dating this woman he just met at college. She is from the middle of nowhere and had a male classmate that went to school in New York City, NYU if I recall correctly.
Long story short, She and my friend were in the very early beginnings of a relationship when she suddenly can't be contacted for New Years Eve. After asking around he learns that she flew to New York. Why? Nobody knows.
Around the 4th of January she suddenly turns back up and she is very honest about what happened.
This male classmate lost a parent and when she gave condolences he offered to fly her to NYC for New Years. They'd hang out, go to the ball dropping, etc. She agreed as he covered the expenses.
Well, and I'm sure you know where this is going, she outright admits to sleeping with him. My friend thinks it's odd but she thinks it's fine as they weren't officially together yet.
My friend is put off by this but not angry with her...until she says something while trying to explain herself.
She tells him that she "felt obligated" to sleep with him. Why?
A) He just lost someone close to him.
B) He paid for her trip to NYC.
Understandably, that's what made him angry because it's such a weird justification. She had to have known that getting on the plane meant she was going to fuck him and she still did it...but then called it an obligation, lol. She and the classmate were never a couple beforehand, btw! Not before or after (as far as I know).
Point being, it sounds crazy and I don't think I've ever hooked up out of "obligation" or a woman feeling pity for me but it does happen...and, like Anna, they don't think it's weird or wrong.
She genuinely thought that my friend would be ok with going forward after hearing that. Shoot, even I spoke to her personally while she tried to get a hold of him after that and even to me she insisted that he shouldn't be mad about it. I had to tell her that he was never mad about what she did but how she justified it, that it came off as weird and a massive red flag. Last I ever spoke to her.
So, while Anna may or my not do things out of pity or a feeling of obligation, it does happen IRL. It absolutely does happen. Some random dude out there fucked his old HS classmate out of pity and dropping a few hundred on a round trip plane ticket lol. Those alone got him laid as even she admitted she "didn't like him like that". It wasn't an old flame, just a dude she knew that was sad.