Was Cain a descendant of the serpent, through Eve? In an article by Ben Heath titled Sex Affair in the Garden of Eden, he claims that the sin for which God expelled Adam and Eve from the garden was sexual intercourse with the serpent. 

Heath goes on to say that Cain and his descendants are literally physical descendants of the serpent, who he interprets as Satan. The doctrine seems to hold a central place in his other teachings. 

Consider 2 Corinthians 11:2-3:


  For I feel a divine jealousy for you, since I betrothed you to one husband, to present you as a pure virgin to Christ. 3 But I am afraid that as the serpent deceived Eve by his cunning, your thoughts will be led astray from a sincere and pure devotion to Christ.


Heath claims that when Paul uses words translated to deceived and beguiled in this and other verses, a better translation is the word seduced, in the sense of tempting to sexual immorality. 

In addition, this website offers an apology of the doctrine, which it calls the serpent seed doctrine, saying:


  You see, those who denounce the Serpent seed teaching as heresy not only deny the teachings of our Lord Jesus Christ, but they also miss out on the first prophecy of the entire Bible. 


According to mainstream Evangelical Protestant denominations like the Southern Baptist Convention, the United Methodist Church, the Assemblies of God, the Presbyterian Church, and the Episcopalian Church, is the serpent seed doctrine a valid interpretation of Genesis 3 and subsequent verses in the New Testament about Eve and the serpent?
I don't have enough information to speak about the Southern Baptist Convention, where each congregation is a separate entity, and some could accept what the OP cites as the "serpent seed" doctrine, nor can I address the issues of the Assemblies of God.

I do have some familiarity with the beliefs of the Methodist and Presbyterian Churches, and am a practicing Episcopalian, and I can say that these groups most emphatically reject this doctrine. Particularly as it directly contradicts Genesis 4:1, which specifically states 


  "Adam knew Eve and she conceived and bore Cain."


And given this verse, the serpent seed doctrine is going to be rejected by the Eastern Orthodox, and Roman Catholic churches, too.