Chazal say (Avodah Zarah 22b) that the snake had relations with Chavah. Does the Gemara elsewhere ever fault her explicitly for this? If not, why not, as she was already commanded in it (Sanhedrin 58a, from Bereishis 2:24).
There are two possible issues here:
1) Did Chava commit adultery against Adam by having relations with the snake?
2) Did Chava commit bestiality by having relations with the snake?
Now of course she did commit these things according to Gem' A"Z 22b. However, the question is if she can be faulted for breaking the law when she did?
1) Adultery did not take place since the law of adultery is only broken when the wife cheats with another man, not an animal. In other words, the guy might be a snake, but he has to at least be a guy. Here the snake is simply an animal so adultery doesn't legally count.
2) Bestiality? This may be more complicated, but she seems to not have violated the "commandment" of bestiality either.
The Gemara on the bottom of Sanhedrin 56a tells us that the sons of Noach were commanded in 7 mitzvos. Sexual transgressions is one of them. The Gemara on 56b then goes on to explain that Genesis 2:16 is the source. Each word is explained by R' Yochonon to apply to one of the 7 in general. In the next verses, Adam names the animals and after meeting Chava says (Gen. 2:23)"This time" it is... flesh of my flesh etc. Rashi brings the Gemara of R' Elazar from Yevamos 63a, which explains that the words "This time" means that Adam first had relations with the animals in an effort to seek a mate, but it did not satisfy him.
If so, Adam was not phased at all that Hashem had told him the animals were forbidden for relations??!
It certainly seems, that the verse in 2:16 as well as the verse expounded in Sanhedrin 57b and 58a (Gen.2:24) are Drashah to provide sources for prohibiting sexual transgressions for B'nei Noach. However, we do not know when those 7 prohibitions took effect upon Adam and his descendants. They certainly did not all take effect at the moment of the verse, since one of the laws is not to eat the limb from a living animal. This was not in effect until Noach earned the right to eat meat. Therefore Adam was a vegetarian. Also, the verse (2:24) "Therefore man should leave his father and his mother...", (which the Gemara explains to forbid one from marrying their mother etc.) does not apply to Adam and Chava, as Adam and Chava had no father or mother!
But we do have the language of the Medrash Rabbah 19:6, on Genesis. "And they perceived that they were naked: Even the one mitzvah that they had was stripped from them..."
As we see, Adam and Chava only had 1 mitzvah in the garden, not 7 plus 1.
Therefore, the verses expounded in the Gemara are sources but do not create prohibitions upon Adam in the garden. Even if you did say that they did create such prohibitions, we do not find anywhere that Hashem warned them what the punishment would be. However, Hashem does warn them about the punishment for eating the fruit. So they only had one mitzvah in the garden.
Alternatively, The Gemara in Sanhedrin 74b says that Esther was not guilty of having relations with Achashverosh since a woman is considered "karka olam" (ground of the world = passive surface) in the act of relations. She is passive and does not perform an act. She could be culpable if she entices the man or if she physically brings the man etc. But if she was passive, then there is no act on her part to make her guilty.
Rashi to Gemara Shabbos 146a says that the fact the snake cohabited with Chava is learned from Gen. 3:13 "הַנָּחָשׁ הִשִּׁיאַנִי וָאֹכֵל.." ("The snake enticed me and I ate.") Rashi explains that the Hebrew "hishiani" is related to "nisuin" or marriage. Therefore the word teaches us a drashah for a double meaning. The snake enticed her to eat the fruit, and the snake "married" her. Since the verb has the snake doing an act upon her, we can assume that Chava did not wish to perform adultery or bestiality, and was passive or even resisting. If so, she is "karka olam" and not responsible.