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.
