Evolutionary theory predicts that assortative mating is crucial for sympatric speciation by generating reproductive isolation between diverging populations. Here, we investigate the potential of form-assortative mating, an assumed mating pattern in lampreys, for sympatric speciation. By continuously recording mating activity between anadromous and freshwater-resident forms of L. camtschaticum that greatly differ in body size, we show that lampreys tend to mate with individuals of similar size in experimental conditions. However, we highlight that this pattern does not result from a choice of same-form partner but is the result of the simultaneous action of a preference of males—whatever their size—for large anadromous females, a higher competitive ability of aggressive males and physical constraints on heteroform pairs. Moreover, we do not advocate that sympatric speciation, as the sole consequence of form-assortative mating through sexual selection, is a plausible mechanism for the diversification of lampreys as a significant number of sneaking behaviors were observed in freshwater-resident males toward large anadromous females. Broader attention should be given to mechanisms other than the sexual selection that may lead to form-assortative mating, such as variations in the spatial or temporal distribution of alternative forms during reproductive season.