I think the line "You're mine now" is being taken a bit differently than it's presented, without really considering the context or the tone of voice Mew is likely delivering it in. The look on her face doesn't indicate yandere possessiveness, nor does the continuation of that line which a lot of people aren't including, "...and I'm yours...okay?" Hell, she even adds the "okay" to make sure that sentiment is acceptable to Donut and she feels the same.
If anything, I think this is less "we fucked, so now you belong to me", and more a continuation of the end of the previous chapter, where they both unequivocally state their commitment to each other, which has been a theme for a while in the series. Donut doesn't belong to her (and vice versa) because they had sex, the sex was the result of them making that mutual commitment.