I am 19 years old, i am african american, i have had a decent amount of sex partners in my life so far, and this problem has never occured until last night, last night was the first time i have had sexual relation with this girl, my penis was very hard the entire foreplay, but when it was time to put on the condom, it just dropped, i eventually got it going like a half hour later but it was really embarrasing, and the next morning it happened again, what is the matter? i smoke marijuana occasionally is that the problem?