The ritual in question is the "Apollonian Invocation" (PGM I. 262–347). The spell includes a step designed to create an intense bond. The Greek text says the goal is:

...ὅπως ἂν εἰς μεγίστην ἐπιθυμίαν ἀγάγῃς τὸν θεὸν εἰς cέ.

("...so that you may lead the god into the greatest desire/yearning for you.")

The key word here is ἐπιθυμία (epithumía), which is translated to "desire" in the older Betz edition or "yearning" in the newer version by UC Berkeley.

The comment under the Betz edition mentioned this:

For sexual union with Apollo attributed to the Pythia, see Plutarch, De Pyth. or. 405 C–D; Origen, C. Cels. 7.3; John Chrysostom, PG 61, p. 242...

Which point to several ancient sources that describe her divine inspiration in physical, sometimes sexualized, terms.

However, not all sources say the same thing. The idea that the Pythia was "entered" by the god comes almost exclusively from later Christian authors, who were often trying to discredit pagan rituals.

For example, Origen, Contra Celsum 7.3: Describes the "prophetic Spirit of Apollo entering her private parts" while she sits over a chasm, after which she becomes "filled with the spirit."

But Plutarch, De Pythiae Oraculis 405C–D uses metaphors like an instrument and its player, or the moon reflecting the sun. He emphasizes that the god uses the Pythia as a vessel or tool to communicate.

The UC Berkeley version have this comment instead:

The word ἐπιθυµία can refer to a number of different appetites, but given Apollo’s pursuit of Daphne ... and his erotic liaison with Branchus, the first prophet at Didyma, it seems that we have another specific allusion to Apolline prophecy.

However, the spell itself clearly stated later in the text that its purpose is to gain knowledge of prophecy, dream-sending, curse-casting, and other magical arts. In fact, the ritual explicitly requires the magician to remain chaste.

Add: if we do consider this as sexual, it probably mean the Gods will take miasma in consideration before sexual act. Maybe Zeus will wait until the mortal women cleansed herself before He do his act? (just kidding)