It's happened to me before.
I can attribute it to several things in my case.
1) Drunk to much
2) was not turned on by the women
Now my belief is that your problem occured because you had perfomance anxiety (sp). You don't need viagra - your not even 20 yet.
I don't think it's attributed to anything your not doing - it's just in your mind. Your probably concerned with the performance more than anything else.
But, let me suggest something to add to your diet.
Zinc - helps maintain erections.
Blueberries - there's a chemical in Blueberries that enables men to maintain a strong errection - I recomend everyone eat these.
Brazil nuts - there's also a chemical here that helps maintain healthy sperm.
1) To answer other questions. It's possible to catch AIDS from oral sex should you have an open wound in your mouth.
2) it is very easy to catch an STD (e.g. HERPES) from oral sex. Should the woman have an outbreak at that time or is shedding.
Educate and read up on both AIDS and STD's BEFORE YOU HAVE SEX WITH ANYONE.
If you don't know how you catch these things - then in my opinion your not mature enough to have sex.
Would you sit in and drive a $100,000 car if you didn't know how to drive?
Next thing, when your in a the "Going to have sex" phase with this woman, relax.
Give her a massage, explore her body, take the pressure of you and give her attention. This does the following.
1) Now your focusing on her. Your massaging her (naked of course) fondling her, feeling her and playing with her. She's getting lots of action here, and your not in a pressure situation.
2) you get to look at her naked. As you explore her body - it will relax you, and I bet you'll get a stiffy when your playing with her.
3) She'll enjoy the **** out of it. You can even go down on her and give her the big 'O' - she's not going to think you can't perform, in fact she'll probably love it - and it'll set you apart from the other 19 yr old's who have fvcked her, and are probably all "come in a flash" - "let me just get it in" guys.
Have fun. If you not ready to fvck her, don't force it. Do it on your time.