Herge once claimed
in an interview that sentimentality had little place in Tintin's stories. They are mainly about men getting into all sorts of "misadventures rather than adventures" and "mocking women would not be nice". He did not consider it funny to draw women slipping on banana peals - though with men it was another matter.
Another factor may have been the general attitude of the time, in continental Europe, that women characters had little place in comics aimed at boys. As both a strip and as a character, Tintin was actually quite typical in being a male character with male friends and only very distant female acquaintances. These comics were for pre-teen boys (adolescents and younger) who were not seen to be old or mature enough to be chasing girls. If women did play an important role in the stories then it was made clear that their relationship with the male heroes was strictly platonic. The comic male characters Spirou and Fantasio, for example, sometimes shared adventures with a girl called Secotine but there was no indication of romance and Fantasio in particular saw her as pesky nuisance and a rival - they were both reporters after the same story.
This changed in the 1960s when female characters started to play more important roles in strips and even appeared as the titled characters. "Castafiore Emerald" with Bianca, Irma and the gypsy girl can be seen as an early example.
These days it is unusual if the main male character in a comic does not have a girlfriend - eventually maybe even a boyfriend in the full sense of the word, but that is another issue.