I understand boundaries, but I must disagree that you were wrong to "snoop," accidentally or otherwise. If your relationship has progressed to the point where you are putting up with his drunken crude texts - without apology - then you are intimate enough to view his facebook account - messages and all.
Granted he was blitzed, but he was speaking how he really feels - and you are right to have some insecurities stemming from this conduct. He apologized to the girl he texted, but where is his apology to you? Does he not realize that he has hurt you, your relationship, and his character with this behavior?
Lastly, he does need counseling. Regardless of his opinion about counseling, being an ISTJ, or anything else, an objective third party can help him get himself straightened out. Multiply this conduct times ten to get an idea of where this relationship will lead the two of you if this conduct is left unchecked.
For me, red flags are everywhere.