"Detumescent" refers to the process of becoming less rigid or swollen. Antonyms of detumescent could include "erect," "rigid," "swollen," "inflamed," "bloated," "turgid," and "puffed up." These words suggest a state of firmness, fullness, stiffness, and swelling that contrasts with the shrinking or softening implied by detumescent.
Other antonyms may include "deflated," "contracted," "collapsed," "shrunk," "empty," and "flat," as they indicate a loss of volume, pressure, or substance. In general, the opposite of detumescent is something that is firm, hard, full, and erect or something that is flaccid, shrunken, or empty.