I can imagine plenty of reasons that are a far cry from the constant bachelor party atmosphere that you imagined. Consider that a single serial rapist could impregnate dozens of black slaves all by himself without the societal approval that you imagine. And once you have mixes-race children in the pool of slaves, those white genes will get passed around. And I do note that most place people living in America today, while they often have some European heritage, rarely seem to have anything close to the European heritage that your constant sex party scenario would produce.
You are also ignoring another reason why many blacks have white genes. Black people and white people, gasp!, actually married each other from time to time. In fact, one of my Jamaican friends in nearly as Scottish as I am and it was entirely through marriage (his grandmother on one side and great-grandmother on the other side were both white) and he has, gasp! married a white woman. But thanks to the "one drop" rule of race, all of those mixed-race children wind up being considered black so they will go on to marry blacks and keep those genes in the black gene pool.
If you think that white slaveowners and their male friends and family weren't enjoying their female slaves, you're hopelessly naive.
Do you have any evidence that this was as widespread as you are suggesting? If not, then I think you are simply making up facts to support your position. I call that "hopelessly grasping at straws". Really, this is exactly what the feminists tried to pull when they claimed that more women were beaten on Superbowl Sunday than any other day of the year without any evidence to support that assertion. I suppose you are going to claim that what really matters is that you are "raising awareness", right?
Mixed race marriage was never illegal in the Caribbean so you're right about that.