Underwear under padded cycling shorts is not a big no no. I wear seamless, form fitting and breathable underwear under my cycling shorts all the time. Underwear isn't going to do anything to help the OP's sweat, bacteria and taint odors but other advice has been offered for that. I'm only chiming in because the Internet has propagated a slew of rules not just for cycling that people hold as a universal truth.
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"It is the unknown around the corner that turns my wheels." -- Heinz Stücke