No, not at all. The reason why Greek gods sometimes interact sexually with mortals in animal form has no definite answer, that is to say none that would be universally accepted. There are a couple of arguments that can be made, though; currently I find the second more persuasive.
It could be simply a storytelling technique so that Zeus can avoid the notice of his wife, Hera, who would be jealous if she saw him having sex with other people. In support of this argument, there's the story that Zeus changed Io into animal form (a cow) to conceal her from Hera. Against, there's the fact that Poseidon and Demeter do this too on one occasion (a couple of late sources report that they have sex in horse form and bear the primal horse Areion).
The animal-form stories could have something to do with theriomorphic deities in proto-Greek religious traditions. That is to say, gods that conventionally appear or are depicted in animal form: this is not how the Greeks themselves normally depicted their gods, but is something that is more familiar from Egyptian mythology (e.g. Anubis with his jackal's head, Horus with his hawk's head). There is some indication that early Greek mythology had theriomorphic elements; but this is a very poorly studied area. This business of sex in animal form is one of the indications; another is the practice of animal sacrifice (but there are multiple possible origins for that practice); another is that certain animals have strong associations with particular deities (Zeus-eagle, Athena-owl, Poseidon-horse, Hera-peacock, etc.); and another is the survival of theriomorphic epithets for some gods in traditional poetry, specifically "ox-faced Hera" and "owl-faced Athena" in the epic tradition, and Dionysos' association with bulls in some iconography and cult hymns (another possibility is Lykeios Apollōn, which could be either a reference to a cult in Lycia or could mean "wolf-like Apollo").
We simply don't know to what extent proto-Hellenic religion was theriomorphic and/or totemic. Explaining the theriomorphic elements as borrowings from the Near East doesn't cut any ice, because it's not hard to find examples in other cultures that had no contact with Mesopotamia/Syria (think for example of Loki's sexual encounter in horse-form). Bull deities are well attested in Bronze Age Crete and Neolithic Anatolia. No need, really, to posit Near Eastern influence.
You can probably tell just from the fact that I spend more time on it that I lean towards the theriomorphic interpretation. But as I said, very little work has been done on this. That's because the study of early mythological/religious topics of this kind is fraught with speculation and conjecture. If any decent work has been done on Greek theriomorphism in the last 100 years (later than the ritualists, that is) I'd love to be made aware of it.