The stem of the Greek noun *erôs* "love, desire" is normally *erôt-*, not *er-*. So it should be **erotogenic** or **erotogenous** in English. Cf. *phôs, phôt-* "light", as in *photograph*, not \**phograph*; *erotic*, not \**eric*.

That said, there are forms in Greek that use *erô-* and *era-* as stems, like the verb *eraô/erô*. It's just that *er(a)-* is the ultimate root of all words, but the stem of the noun is normally *erôt-*. Ancient Greek has many dialects and oddities, and some writers even use *erô-* as the stem of the noun in certain forms. So perhaps *erogenous* is defensible, but it's not standard Attic, so it is not the way a normal formation of a Latin or modern word based on Greek is supposed to go.