Hermaphrodites are an actual thing that happens. Some are even born with both parts being functional. In general people call a hermaphrodite with a female body form a futanari because Japan popularised it. There is no implied resistance to testosterone anywhere and if they are born without testicals as most futanari are drawn they wouldnt have an uptick in testosterone anyways. Alternatively their body could adapt other parts of their reproductove system to produce spermatazoa meaning they could produce sperm without needing massive quantities of testosterone, their sperm would be different but its biologically possible.