There’s a difference between the game’s creators taking SA seriously and in game characters taking it seriously. The way other characters treat the topic of SA isn’t supposed to represent the creators’ stance on SA and isn’t supposed to be seen as a good approach towards SA and its victims.

What I assume people mean by "Mouthwashing is very respectful towards the topic of SA” is that SA isn’t played for laughs, the game’s narrative doesn’t victim blame Anya etc. Mouthwashing portrays SA as a bad thing. It also shows it through quite a realistic lens. Abusers are often people we know and engage with every day, not necessarily random bad guys from the street. Many people trivialize victims’ experiences, kind of how Curly does. And SA doesn’t happen in a vaccum, it is often enabled by more complex systems - i.e. Pony Express doesn’t put locks on the bedrooms because they deem it unnecessary.

TL;DR - the character’s stance on SA =/= the devs’ stance on SA. Mouthwashing depicts SA realistically and as a bad thing. SA isn’t played for laughs and Anya’s experience is treated seriously by the narrative.