You are probably best to put any new topics under "New to Sex", I doubt you have a sexual disorder. I'm assuming that any other time you function fine such as while; masturbating, oral, or a hand job.
Alcohol, medications, & drugs will cause you to go limp and not be able to climax. Not a good mix. If you had a condom on it causes decreased sensation and can be culprit, nerves or anxiety can do the same. Additionally, the vaginal cavity it not the same grip as you are accustomed to during masturbating. The speed, rhythm, and sensation during intercourse is also different which can be a cause.
Here is a link to a place to begin reading to help you:
http://www.sexinfo101.com/forum/new_...ed_topics.html