its a tricky situation. if its perfectly innocent and you make a big deal of it then you look like you dont trust your partner and that can start more problems. on the other hand its unlikely that the texts are completely innocent, not impossible, but unlikely.
i think there is a big difference between txt flirting and actually meeting up. if you meet up and dont do anything it is still wrong when you're in a relationship, but txting is harmless as long as it doesnt lead to anything else.
when you're in a relationship you like to think that your partner only has eyes for you, but in reality if they fancy you then they must fancy other people too, it is human nature. its ok to fancy someone else, its harmless flirting with them, but going any further is wrong imo.