Since when did we judge themes based solely on Monster Cards? For example, Crystal Beasts would be useless without their support cards. Venom monsters practically revolve around Venom Swamp, which for one effectively makes the Tribute Venom monster a 2600 ATK monster against 1 other monster immediately, and further weakening it (and all other monsters) later. And there's a whole load of other support lying around too, much of which comes from TAEV too.
And Vennominon and Vennominaga are unaffected by Venom Swamp, so they still fit into the theme. And Reptiles now have plenty of support, for one, being able to bring out Vennominon if you take any Battle Damage at all involving a Reptile (Damage = Reptile). It still all works well together, and can be rather effective when played right, and I'm also off topic.