beta
I think the author chose to show the confession through a sex scene because so far in the series, every sexual encounter (especially of a penetrative nature) has been violent and non-consensual.
So, I feel the author’s intent was to take the same situation, same frame of event (that is, the act of having sex) and try to portray how things have changed for both of them.
Of course she could have also chosen to not go about it this way and could have played the scene differently. But this personally feels more in line with the characters to me (and also considering how those violent acts are seeped in the history of their relationship)