Don't worry, the same thing happened to me the first two times I tried doing the deed. The third time was awesome! It's been good almost every time since then. Most women understand that a penis can't necessarily jump to life on command. If you think your girl is dissappointed in the erectness of your penis, just make up for it with other means of stimulation. Use your imagination and don't put pressure on yourself or your partner to climax. Just try to have fun and my bet is that she'll be very satisfied. Even if actual penile penitration doesn't happen right away, as the two of you become used to connecting intimately a comfort level will develop that will allow for more "uninhibited" sexual encounters and you'll most likely find your ection difficulties a thing of the past. A bonus is that the two of you will probably learn some great other ways of pleasing each other along the way!

More than likely you were doing nothing wrong--physically; although, morally and legally might be another matter.

What you experienced that resulted in a partial erection is often common to first timers at any age and is known as "Performance Anxiety". The trick is not to worry about it because worrying about it makes matters worse.