All praise is due to Allah alone, and may the Salah and Salam of Allah be upon our Prophet Muhammad, his family and his companions. To answer your question, we say while seeking all the success from Allah: First, I advise you to refrain from getting involved in debates and discussions that you cannot handle because, despite the vast knowledge and strong belief of the righteous predecessors, even they used to avoid debating and arguing with the people of falsehood. In fact, their statements that have been transmitted to us in this regard show that they disallowed people to delve into such matters. Therefore, I strongly advise you to follow their path. They only involved themselves in some debates after they were certain that they could bring great benefit to do so, and they only debated out of necessity. Imam Ahmad (may Allah have his mercy on him) said: “We used to order [people] to remain silent [i.e. avoid debates and arguments], but when we were notified of a matter (doubt and deviation), I felt compelled to stand up and clarify it to refute their doubts.” He also said: “We used to remain silent, until we had to step up and talk. If one finds themselves in such a position, then they must not talk without profound knowledge to protect themselves from the doubts casted by those who liken Allah to His creation and to refute their deviations.” As for the issue of marrying a girl who is still being breastfed, Ibn al-Mundhir said: ‘All the scholars we know agree that the father can wed his virgin young girl, if the groom is competent.” The intended meaning of this statement is to conduct a marriage contract only. As for enjoyment and intercourse, it is only allowed after she reaches an age that makes her suitable for it. Some scholars specified nine years of age as the suitable age, which was specified based on the norms. However, if the girl cannot handle intimacy in that age (9 years), then it is forbidden because it will harm her. As for the statement of al-Khumaini, which you have mentioned, it means enjoying the girl and having intimacy with her because the concept of muta’a (temporary marriage) is based on paying money for the purpose of enjoying a girl, and not to start a marriage relation that has rights and obligations. With that said, it is obvious that there is a difference between what the scholars of Ahlu al-Sunnah said and what al-Khumaini said, may Allah give him what he deserves. And Allah knows best.