Ibn Umar misunderstood (the Qur'anic verse, So come to your tilth however you will)--may Allah forgive him. The fact is that this clan of the Ansar, who were idolaters, lived in the company of the Jews who were the people of the Book. They (the Ansar) accepted their superiority over themselves in respect of knowledge, and they followed most of their actions. The people of the Book (i.e. the Jews) used to have intercourse with their women on one side alone (i.e. lying on their backs). This was the most concealing position for (the vagina of) the women. This clan of the Ansar adopted this practice from them. But this tribe of the Quraysh used to uncover their women completely, and seek pleasure with them from in front and behind and laying them on their backs.

When the muhajirun (the immigrants) came to Medina, a man married a woman of the Ansar. He began to do the same kind of action with her, but she disliked it, and said to him: We were approached on one side (i.e. lying on the back); do it so, otherwise keep away from me. This matter of theirs spread widely, and it reached the Messenger of Allah (ﷺ).

So Allah, the Exalted, sent down the Qur'anic verse: Your wives are a tilth to you, so come to your tilth however you will, i.e. from in front, from behind or lying on the back. But this verse meant the place of the delivery of the child, i.e. the vagina.

Anas bin Malik said Among the Jews when a woman menstruated, they did not eat with her and drink with her and did not associate with her in their houses, so the Apostle of Allaah(ﷺ) was questioned about it. Hence, Allah the Exalted revealed “And they ask you about menstruation,. Say “It is harmful, so keep aloof from women during menstruation till the end of the verse. The Apostle of Allaah(ﷺ) said “Associate with them in the houses and do everything except sexual intercourse. The Jews thereupon said “This man does not leave anything we do without opposing us in it. Usaid bin Hudair and Abbad bin Bishr came to the Apostle of Allaah(ﷺ) and said, Apostle of Allaah(ﷺ) the Jews are saying such and such. Shall we not have intercourse with them during their menstruation? The face of the Apostle of Allaah(ﷺ) underwent such a change that we thought he was angry with them, so they went out. They were met by a gift of milk which was being brought to the Apostle of Allaah(ﷺ) and he sent after them, whereby we felt that he was not angry with them.