Allah said,

وَكَيْفَ تَأْخُذُونَهُ وَقَدْ أَفْضَى بَعْضُكُمْ إِلَى بَعْضٍ ...

And how could you take it (back) while you have gone in unto each other!

how can you take back the dowry from the woman with whom you had sexual relations and she had sexual relations with you!

Ibn Abbas, Mujahid, As-Suddi and several others said that;

this means sexual intercourse.

... وَأَخَذْنَ مِنكُم مِّيثَاقًا غَلِيظًا ﴿٢١﴾

and they have taken from you a firm and strong covenant!

The Two Sahihs record that;

the Messenger of Allah said three times to the spouses who said the Mula`anah;

اللهُ يَعْلَمُ أَنَّ أَحَدَكُمَا كَاذِبٌ، فَهَلْ مِنْكُمَا تَائِبٌ ؟

Allah knows that one of you is a liar, so would any of you repent?

The man said, "O Messenger of Allah! My money,'' referring to the dowry that he gave his wife.

The Messenger said,

لَا مَالَ لَكَ، إِنْ كُنْتَ صَدَقْتَ عَلَيْهَا فَهُوَ بِمَا اسْتَحْلَلْتَ مِنْ فَرْجِهَا، وَإِنْ كُنْتَ كَذَبْتَ عَلَيْهَا فَهُوَ أَبْعَدُ لَكَ مِنْهَا

You have no money. If you are the one who said the truth, the dowry is in return for the right to have sexual intercourse with her. If you are the one who uttered the lie, then this money is even farther from your reach.

Similarly Allah said;

وَكَيْفَ تَأْخُذُونَهُ وَقَدْ أَفْضَى بَعْضُكُمْ إِلَى بَعْضٍ وَأَخَذْنَ مِنكُم مِّيثَاقًا غَلِيظًا ﴿٢١﴾

And how could you take it (back) while you have gone in unto each other and they have taken from you a firm and strong covenant) (Be kind with women, for you have taken them by Allah's covenant and earned the right to have sexual relations with them by Allah's Word.