Toward the end, we read thus (my emphasis):
"O Merlin, though you do not love me, save,
Yet save me!" clung to him and hugged him close;
[...] she called him lord and liege,
Her seer, her bard, her silver star of eve,
Her God, her Merlin, the one passionate love
Of her whole life; and ever overhead
Bellowed the tempest, and the rotten branch
Snapt in the rushing of the river-rain
Above them; and in change of glare and gloom
Her eyes and neck glittering went and came;
Till now the storm, its burst of passion spent,
Moaning and calling out of other lands,
Had left the ravaged woodland yet once more
To peace; and what should not have been had been,
For Merlin, overtalked and overworn,
Had yielded, told her all the charm, and slept.
This passage feels like there is much more happening than a simple social engineering attempt in a rainstorm. Rather, the imagery seems consistent with Vivien bringing Merlin to sexual climax - the imagery of rushing, coming, and sleeping afterwards.
Is this a reasonable interpretation of the passage, or am I missing something?