Opossums are unique in that they are facultative parthenogenetic. This means that they can reproduce through asexual reproduction, which doesn't require a male. In this case, the female opossum's egg develops into a clone of itself.
However, opossums can also reproduce sexually with a male opossum. In this case, they follow the typical mammalian reproductive process.
So, while a male opossum isn't necessary for a female to reproduce, it's not impossible for them to reproduce sexually.