See Tosfos Boba Metzia 58b, and Eruvin 19a (And Gemora). The bottom line is that sleeping with a shiksa 'in and of itself' is not such a big sin, but the fallout is horrendous. The fallout is that Avrohom Avinu does not see your "bris", and he does not recognize you as part of his offspring, and he does not pull you out of 'gehenom'.
For example, if you slept with a married woman, and you deserve a full 12 months in gehenom- if you did not sleep with a shiksa- Avrohom Avinu will pull you out right away, and you will not sit there 12 months. But if you slept with a shiksa "forget about it", you're finished!!
The way Rabbi Akiva Tatz explains this it's like G-d saying "listen, you're a bad boy and you deserve "this and this". However, since you are your fathers son, and I love your father, I just can't do this to your father". But if you slept with a shiksa, "what son?" "what father?"
BTW, if you're an Apikores you will never get out of hell (Tosfos). Now what happens if an Apikores never slept with a shiksa? Why? You know of one? (His "bris" may be bad anyway. Who does he have a bris with anyway, Darwin?)
Hay, Bible Critics!! You can scream all you want "I don't believe" "I don't believe" but it ain't gonna help you nothing. You really think G-d would run away just because you woke up one morning and said "I don't believe?" Who the hell do you think you are anyway? You're just a little pisher who probably forgot to take his pills.
P.S. Tosfos says you should do Tshuvah. Tshuvah always helps. So the next time she calls don't do it.