Intromittent organs (IOs) have evolved many times within the animal kingdom and are remarkable for their extravagant morphological diversity. In some species, an IO is built of tissues with reproductive system antecedents, but others have modified fins, tentacles, or legs for intromission; anatomically, IOs can include combinations of stiff tissues, extensible tissues, and muscle. IO behavior during copulation is also diverse: males in some taxa reorient or protrude their genital structures, others inflate them and change their shape or use a combination of these behaviors. But despite morphological and behavioral variation, all IOs share a function: intromission during copulation. Typically, IOs are stored in a non-reproductive state and shifted into a distinct reproductive morphology immediately before or during copulation. I propose that the timing of this shift constrains the subsequent mechanical behavior of the IO. If the IO shifts to its reproductively relevant morphology before copulation, its morphology will be optimized for high flexural stiffness to allow it to effectively enter female reproductive structures. IOs that shift into a reproductively relevant morphology during copulation will be more likely to have tissues with material properties that permit them to fill female reproductive spaces, but produce structures with lower overall flexural stiffness.