@lonefire
My guess is she wanted the MC to claim her in a way and erase what the molester(the guy the was groping her in the festival) from the last chapter did by leaving his mark (the MC kisses). I hope that makes sense to you (I could elaborate if needed).
@stanfromis_9
I don't that it's a toxic relationship. Next chapter will add more context on why she's acting the way she is. Without giving any spoilers, FMC is working through something and I think it's good the MC is there to support her. Things may seem that that they are just doing naughty with each other for the say of being naughty but the next chapter will show that all those actions in way, is just them licking each other wounds in a sense.