Mothra would eventually become restless and eager to join in the fun, because as much as she loves to see her alpha work, she loves to be on the receiving end even more. And knowing that, Goji would eventually pull out of Ghidorah just as they are having yet another intense orgasm (and at that point, their climaxes would already be at the double digits) and while he's not quite done with them just yet but wants his Queen to join in. So, he looks towards Mothra and motions his head, telling her to come closer, he wants her to guide him back in...
(Pretty sure this is how the sex happens)
He's going to be locked in his feral "breed the living daylights out of anything that presents itself" musth state for a while yet, especially once Mothra rushes in and begins worshiping his length. Feral!Goji grins down at the mouth-watering sight of his tiny sex-drunk mate rubbing herself all over him, squeezing her legs around his girth and peppering it with licks and kisses.
He starts getting impatient though, and growls his barely coherent intent ("GONNA FUCK YOU... GONNA BREED YOU... MINE...") and she makes the neediest noises possible and gets into position. But before they can start the fun, Goji leans down so their faces are somewhat level and opens his mouth for her to kiss (or at least the Mosugoji equivalent of kissing). No room for technique or finesse in these kisses, just two alphas sloppily claiming each others mouths. Then once she's distracted by his tongue, that's when he slips inside her and stretches her out.