Each case is individual and should be analyzed separately. In some cases first contact really happens under influence of alcohol or drugs (amphetamine, ecstasy, etc.). In small doses alcohol have stimulating effect. In bigger doses though – it suppresses senses, in the beginning it’s just harder to ejaculate, later there’s problems with erection and then no erection at all. Situation is – there’s a willing but no possibility.
Speaking about alcohol and other toxic substances the most important is that people loose self-control under their effect, even basic caution is taken down so people are putting themselves under a big risk of infections with STD including HIV/AIDS and of course unplanned pregnancy.
Mostly the reason of failures is stress. So to achieve a real pleasure from first sexual contact and escape the failure there are some advices which should be taken into account. The best is if it can be done at well known place like home for instance. The perfect is if you can dedicate as much as possible time for it, whole night for example. It helps for very sexual act as well as to avoid any unpleasant situations, like loss of erection at very beginning of act. Don’t worry, just wait for a while and your “friend” will be up again. But if such a situation happens somewhere outside in bushes then there can’t even be a question about resting and continuing in a while.