Shalom, why isn´t the progesterone pill a problem of hashjatat zera o zera lebatala if it makes a thick mucus wall that does´t let the zera from penetrating.
The prohibition of השחתת זרע is not determined by whether there is a wall or barrier preventing the זרע from entering the uterus. After all, this is the case with a pregnant women and a בתולה and yet with both there is no prohibition. Ultimately, as long as there is no external barrier, such as a diaphragm or condom, there is no issur of השחתת זרע. This is because the תשמיש is done כדרך כל הארץ, in a normal way without interruption. The fact that there are hormones causing various levels of mucus buildup and similar conditions which ultimately prevent pregnancy from taking place, is not in violation of השחתת זרע. Nevertheless, there is a different halachic issue involved. That is the question of מצות פריה ורביה, the obligation to bring a boy and girl into the world. Even after having fulfilled this commandment, a Rav should be consulted before hormonal contraception, as there remains a Rabbinic commandment of לערב אל תנח ידך, which is the mitzvah drabanan to continue to have children even after pru urvu.