Gemara Yevamot 55b (Soncino's Translation)
what, however, can be said, according to him who maintains [that for such an act one is] guilty? - The exclusion is rather that of intercourse with a dead woman. Since it might have been assumed that, as [a wife], even after her death, is described as his kin, one should be guilty for [intercourse with] her [as for that] with a married woman, hence we were taught [that one is exonerated].
So there is a Hava Amina, before the teaching of a redundant verse, that a man who sleeps with a married woman, after her death, is Chayav as if she was alive. Sheer is obviously not a sign that she is still married to him. Following this logic if he has intercourse with his dead mother is it an incest? Obviously not.
In Gemara, each Hava Amina needs to be understood because it contains a rational argument. I don't understand how such a case can be viewed as an adultery. Can someone help me to understand better the Hava Amina?