I am aware that Ramba"m lists the commandment of "Be fruitful and multiply" (Breishit 1) as being the first mitzvah listed in the Torah (See Minyan Hamitzvot #212). However, this mitzvah is not one of the 7 Noahide mitzvoth.
Prior to the flood, there was nothing as the 7 Noahide commandments. Yet, we see that G-d was angry at Kayin for murdering his brother, Hevel. Was Kayin supposed to know that murder was forbidden? There were no explicit commandments as such!
Likewise, mankind (except for Noach and his family) was destroyed in the flood, partially because of idolatrous worship (See Rash"i on Bresihit 6:10). Why were these people punished if there was no explicit Noahide law directing them against idolatry?