One could read these lines as a description of a lead-up to sex.
Labour to admit you (line 6)
This line could be interpreted sexually.
Except you enthrall me, never shall be free, Nor ever chaste, except you ravish me. (lines 13-14)
Finally, we get to the part of the poem where it is difficult to ignore the sexual connotations. Even if enthrall ("fill with delight") doesn't convince you, "ravish" probably does. See the "Detailed Summary," the "Sex Rating" section for more.