Hi Julian,
Unfortunately, you are emotionally further along than this person is, which has led you to have certain expectations that this person doesn’t appear ready to meet.
They haven’t actually committed to a serious relationship with you. Just because you have texted each other often, gone on dates and had sex multiple times over the course of several weeks doesn’t mean that you can lay claim to this person.
Until they choose to be exclusive to you, they are technically still single. They aren’t cheating on you by remaining active on dating apps because the both of you aren’t in an exclusive relationship.