I would not fault yourself in any way for your son's sexual explorations. You mention having had many conversations with him in which you shared both factual information and your beliefs and convictions about sex. The fact that he chose to experiment sexually does not mean that he does not value the sexual beliefs that you shared, nor does it mean that he is an immoral young man. He has chosen to talk to you after having had sex with his girlfriend. By doing so, he clearly wants more dialogue with you.
I understand your dilemma of not wanting to appear to condone or encourage his having sex by giving him accurate information about disease prevention and birth control. Even though you may have religious and moral beliefs against the use of birth control, I think you could say something like, " You know from our past conversations the reasons why I don't think it's right for you to have sex at this time. If and when you do choose to have sex again, I'd like you to be sure to take the best precautions available to prevent any diseases and unwanted pregnancy." After that introduction, ask him what protection he and his girlfriend used. Most teenagers give the impression that they "know everything" about safer sex (there is no such thing as absolutely safe sex) but further discussion about the matter usually reveals that they possess much incorrect information. It may be helpful to have discussions with your son individually and your son and his girlfriend together.
STD's must be part of the sexual dialogue with your teens. Sexually transmitted diseases like herpes, gonorrhea, and chlamydia are most often transmitted through intercourse and are often left out of these parent/child sex discussions because the emphasis is on AIDS.
You can't forbid your 19-year-old son to have sex. If you will not acknowledge or respect your son's considered opinions on sex because they differ from yours, then he will cease talking to you about sexual matters -- that's the last thing that you want to happen. I know that this area is a complex, frustrating, and frightening one but you need to do everything you can to keep those lines of communication open in this area. I'm betting that you will. Thanks for writing.