Pretty normal to assume that when both of them get jobs together and they end up in a relationship together that would mean that Mashiro would stop selling sex.
I doubt the presumably part time job at the cafe pays anywhere near as well, even if we include the doubled income from them both working, and it's best to not to eliminate your main income source on the basis of counting on Makiro being able to hold down a job. I would guess that the cafe also pays worse than the tutor job that she lost.
Something else worth mentioning is that not every relationship operates under the 'presumption of exclusivity' where, once you start dating, it is automatically assumed you will not be sexually or romantically involved with any other person. I have the impression that heterosexual relationships operate more under 'assumed roles' and so it may not occur to this population to talk about it, but, without normative roles to reliably fall back on, LGBT people tend to have to explicitly communicate about stuff like this more.
In my limited romantic experience, we did talk about if we wanted to be monogamous at the beginning of the relationship. I have known some who consider it a requirement to be monogamous, some who consider it a requirement to not be (and may even have other romantic relationships going on in tandem), and some not caring either way as long as there is agreement. Some people would also consider sex work separately, as it's not being done for recreational or romantic reasons, even if they did want to be monogamous.
Plus, it's just better to talk about this and remove room for doubt, not just to take the few minutes to explicitly verify that you're going to do what you were probably going to do anyway (which is still good), but also because everyone has their own idea of what exclusivity means and what their boundaries are. For instance, if someone considered opposite sex friendships off the table in a monogamous relationship, you are better off getting that information from them ASAP so you can decide if you want to commit to that, rather than having it unexpectedly come up after you've invested some months into the relationship.
In any case, it would be presumptuous to assume someone is necessarily going to change careers because one wanted to date them. The person being approached might reasonably think the other was was okay with them doing that work, given that the other approached while knowing about that income source. Considering Mashiro and Makiro haven't even talked about it yet, it makes more sense that Mashiro will continue to do it for the time being.
