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.  

إِنَّ ابْنَ عُمَرَ وَاللَّهُ يَغْفِرُ لَهُ أَوْهَمَ إِنَّمَا كَانَ هَذَا الْحَىُّ مِنَ الأَنْصَارِ وَهُمْ أَهْلُ وَثَنٍ مَعَ هَذَا الْحَىِّ مِنْ يَهُودَ وَهُمْ أَهْلُ كِتَابٍ وَكَانُوا يَرَوْنَ لَهُمْ فَضْلاً عَلَيْهِمْ فِي الْعِلْمِ فَكَانُوا يَقْتَدُونَ بِكَثِيرٍ مِنْ فِعْلِهِمْ وَكَانَ مِنْ أَمْرِ أَهْلِ الْكِتَابِ أَنْ لاَ يَأْتُوا النِّسَاءَ إِلاَّ عَلَى حَرْفٍ وَذَلِكَ أَسْتَرُ مَا تَكُونُ الْمَرْأَةُ فَكَانَ هَذَا الْحَىُّ مِنَ الأَنْصَارِ قَدْ أَخَذُوا بِذَلِكَ مِنْ فِعْلِهِمْ وَكَانَ هَذَا الْحَىُّ مِنْ قُرَيْشٍ يَشْرَحُونَ النِّسَاءَ شَرْحًا مُنْكَرًا وَيَتَلَذَّذُونَ مِنْهُنَّ مُقْبِلاَتٍ وَمُدْبِرَاتٍ وَمُسْتَلْقِيَاتٍ فَلَمَّا قَدِمَ الْمُهَاجِرُونَ الْمَدِينَةَ تَزَوَّجَ رَجُلٌ مِنْهُمُ امْرَأَةً مِنَ الأَنْصَارِ فَذَهَبَ يَصْنَعُ بِهَا ذَلِكَ فَأَنْكَرَتْهُ عَلَيْهِ وَقَالَتْ إِنَّمَا كُنَّا نُؤْتَى عَلَى حَرْفٍ فَاصْنَعْ ذَلِكَ وَإِلاَّ فَاجْتَنِبْنِي حَتَّى شَرِيَ أَمْرُهُمَا فَبَلَغَ ذَلِكَ رَسُولَ اللَّهِ ﷺ فَأَنْزَلَ اللَّهُ ﷻ { نِسَاؤُكُمْ حَرْثٌ لَكُمْ فَأْتُوا حَرْثَكُمْ أَنَّى شِئْتُمْ } أَىْ مُقْبِلاَتٍ وَمُدْبِرَاتٍ وَمُسْتَلْقِيَاتٍ يَعْنِي بِذَلِكَ مَوْضِعَ الْوَلَدِ  

[Machine] Indeed, Ibn 'Umar and Allah forgive him and his. This village belonged to the Ansar, who were polytheists along with the Jews, who were people of the book. They used to consider themselves superior and thus many would imitate their actions. It was the custom of the people of the book that they would not approach women except in a certain manner, as a means of concealing the woman. This village, being from the Ansar, adhered to this practice. They would explain and extensively enjoy the company of women, engaging with them face to face, from behind, and lying down. When the Muhajireen arrived in Medina, a man from among them married a woman from the Ansar and began to engage in these practices with her. She objected and said, "We were used to being approached in a certain way, so adhere to that or keep away from me until you seek the permission of their Prophet." This was conveyed to the Prophet ﷺ , and Allah revealed the following verse: "Your wives are a place of cultivation for you, so approach your cultivation however you wish." (Surah Al-Baqarah 2:223) This refers to the place of childbirth. This narration is also reported by 'Abdul Rahman ibn Muhammad Al-Muharibi from Muhammad ibn Ishaq, who heard it from Aban ibn Salih, who mentioned it in a similar manner, adding, "After it is in the womb."  

إِنَّ ابْنَ عُمَرَ وَاللهُ يَغْفِرُ لَهُ وَلَهُمْ إِنَّمَا كَانَ هَذَا الْحِيُّ مِنَ الْأَنْصَارِ وَهُمْ أَهْلُ وَثَنٍ مَعَ هَذَا الْحِيِّ مِنْ يَهُودَ وَهُمْ أَهْلُ كِتَابٍ كَانُوا يَرَوْنَ لَهُمْ فَضْلًا عَلَيْهِمْ فَكَانُوا يَقْتَدُونَ بِكَثِيرٍ مِنْ فِعْلِهِمْ وَكَانَ مِنْ أَمْرِ أَهْلِ الْكِتَابِ أَنْ لَا يَأْتُوا النِّسَاءَ إِلَّا عَلَى حَرْفٍ وَاحِدٍ وَذَلِكَ أَسْتَرُ مَا تَكُونُ الْمَرْأَةُ وَكَانَ هَذَا الْحِيُّ مِنَ الْأَنْصَارِ قَدْ أَخَذُوا بِذَلِكَ مِنْ فِعْلِهِمْ وَكَانَ هَذَا الْحِيُّ مِنْ قُرَيْشٍ يَشْرَحُونَ النِّسَاءَ شَرْحًا مُنْكَرًا وَيَتَلَذَّذُونَ مِنْهُنَّ مُقْبِلَاتٍ وَمُدْبِرَاتٍ وَمُسْتَلْقِيَاتٍ فَلَمَّا قَدِمَ الْمُهَاجِرُونَ الْمَدِينَةَ تَزَوَّجَ رَجُلٌ مِنْهُمُ امْرَأَةً مِنَ الْأَنْصَارِ فَذَهَبَ يَصْنَعُ بِهَا ذَلِكَ فَأَنْكَرَتْ عَلَيْهِ وَقَالَتْ إِنَّمَا كُنَّا نُؤْتَى عَلَى حَرْفٍ فَاصْنَعْ ذَلِكَ وَإِلَّا فَاجْتَنِبْنِي حَتَّى شَرَى أَمْرُهُمَا فَبَلَغَ ذَلِكَ النَّبِيَّ ﷺ فَأَنْزَلَ اللهُ تَبَارَكَ وَتَعَالَى {نِسَاؤُكُمْ حَرْثٌ لَكُمْ فَأْتُوا حَرْثَكُمْ أَنَّى شِئْتُمْ} [البقرة 223] مُقْبِلَاتٍ وَمُدْبِرَاتٍ وَمُسْتَلْقِيَاتٍ يَعْنِي بِذَلِكَ مَوْضِعَ الْوَلَدِ وَرَوَاهُ أَيْضًا عَبْدُ الرَّحْمَنِ بْنُ مُحَمَّدٍ الْمُحَارِبِيُّ عَنْ مُحَمَّدِ بْنِ إِسْحَاقَ سَمِعَ أَبَانَ بْنَ صَالِحٍ فَذَكَرَهُ بِمَعْنَاهُ وَقَالَ بَعْدَ أَنْ يَكُونَ فِي الْفَرْجِ