“Moving somewhere more serious” and “not there yet” equals Nothing. Sorry op. The road to hell is paved with good intentions. You’re not officially a couple. You hook up sometimes. That’s all. There is no loyalty expected. Yes, you say you are exclusively having sex, that’s smart, but in no way is that the same thing as your Friend owning you txting monogamy. Maybe your friend even wants to explore the old partner for a romantic relationship or for sex. At that point, you can expect your friend to come talk to you and break it off or get more exclusive. Right now you have zero to get upset over as that level or commitment and monogamy was never established.