A:  A number of factors may contribute to sexual difficulties: 1) physiological factors (certain medical conditions or abuse of alcohol or drugs), 2) cultural influences (negative childhood experiences associated with sexuality or performance anxiety - not able to “measure up”), 3) individual factors (self-esteem, sexual orientation or fear of intimacy), and 4) relationship factors (fear of pregnancy or STDs, unresolved relationship problems).  

Based on the information you give, there is no evidence of a physiological problem - unless, of course, these sexual experiences were preceded by heavy drinking or drug use.  Rather, these circumstances are more likely attributable to psychological factors.  You mention that you were unable to achieve erection during your first sexual contact.  If this experience was traumatic, it may have heightened your concern about future encounters and established performance anxiety.  Anxiety is physiologically incompatible with erection -- it inhibits arousal.  Furthermore, once an individual becomes overly anxious about his sexual performance, he assumes a spectator role.  Rather than being mentally, emotionally and physically present while engaged in sexual activity, he is cognitively detached - distracted by thoughts of  “can I do it this time?”  This detachment can prevent, or cause him to lose, an erection.  The best advice is to put an end to the self-fulfilling prophecy of performance anxiety.  Rather than focus on your performance, focus on the pleasure of your partner.  Remember, sex is much more than just penile-vaginal intercourse. 

Another possible contributor is anxiety related to intimacy.  Masturbation is relatively uncomplicated -- it requires only sexual desire.  Sex with a partner, however, brings up issues of trust, communication, and emotional and physical intimacy.  If you feel threatened by intimacy, the result can be sexual difficulty.  A solution is to find a partner with whom you feel comfortable, relaxed and whom you can trust.