Sex allocation theory predicts that the optimal sexual resource allocation of simultaneous hermaphrodites is affected by mating group size (MGS). Although the original concept assumes that the MGS does not differ between male and female functions, the MGS in the male function (MGSm; i.e., the number of sperm recipients the focal individual can deliver its sperm to plus one) and that in the female function (MGSf; the number of sperm donors plus one) do not always coincide and may differently affect the optimal sex allocation. Moreover, reproductive costs can be split into “variable” (e.g., sperm and eggs) and “fixed” (e.g., genitalia) costs, but these have been seldom distinguished in empirical studies. We examined the effects of MGSm and MGSf on the fixed and variable reproductive investments in the sessilian barnacle Balanus rostratus. The results showed that MGSm had a positive effect on sex allocation, whereas MGSf had a nearly significant negative effect. Moreover, the “fixed” cost varied with body size and both aspects of MGS. We argue that the two aspects of MGS should be distinguished for organisms with unilateral mating.