I don't think the complaints are about Anne being a slut. That is part of her charm! The major complaints seem to be that Anne is out of control. She screws anyone and everyone. (And depending on your path) She gets very angry at the MC if he attempts to say no to any of her behaviors, even when he is completely faithful. That bitchiness makes her very unlikable.
In an attempt to remedy people's hate for Anne, the developer made some retroactive changes to Anne's dialogue. Those changes made it so she wasn't so bitchy. One of the most notable changes was when the MC gets caught getting a blow job from Elena while on their first vacation. Presumably, Anne has been getting all the action (malls, dinner, Nicole, Ryan, Pizza guy x2, Airplane guy, Vacation waiter, Pool guys, and the Beach). Anne had just had sex with Cornell, Randall, and Will. And gives a blowjob to the bartender (Arran) at the pool. Before the update, Anne would get pissed off that you get the blowjob and she would flip you off and runs out to fuck Cornell. She ends up fucking the two guys she later falls in love with instead. After the update, Anne is happy for you and can't wait to hear about how you met Elena. She still goes off to fuck the two guys instead. She just isn't a complete bitch about you getting a blowjob.
I think what makes Anne so unlikable is how she responds when you tell her no, the fact that cheats with your best friend and dad, fucks everything that walks, and gets so angry at the MC — even with high relationship points. She is not much better after the update (she still does all those things) except she doesn't get as angry at the MC for getting some himself.
Later complaints are because the sex quickly becomes mundane and Anne is way too into the guys she is having sex with. This is way worse if you choose a path where Anne is with Ryan, Martin, and Dre.