Hi there. Normally when a person thinks of sex, this is enough stimulation to cause spontaneous erection and discharge from the penis called pre-ejaculate (pre-cum). This is perfectly normal with any kind of sexual stimulation. Usually erectile dysfunction can be noticed only while having sex and not before. So I suggest you consult your Urologist if you aren t able to maintain erection during sexual intercourse. Avoid smoking, alcohol. Eat healthy. Avoid stress in your life. Do Yoga and deep breathing exercises. There are various methods to treat erectile dysfunction but must be confirmed only if you cannot have normal satisfying sexual intercourse. Don t worry now. Exercise daily as this will help improve your stamina.