A feminist would approve of this sonnet, I would think. The sonnet argues for the constancy of love:
O no, it is an ever-fixèd mark
That looks on tempests and is never shaken;
A feminist would therefore approve of love that was everlasting. Plus, it argues that love does not fade just because a woman ages (in this sonnet, it is a woman):
Love's not Time's fool, though rosy lips and cheeks
Within his bending sickle's compass come
Even if a woman ages and no longer has "rosy lips and cheeks" - love should be constant, and not come and go with youth.
Love is forever and should not end or grow old with time:
Love alters not with his brief hours and weeks,
But bears it out even to the edge of doom
"Until death do us part" - so to speak. In the end, Shakespeare says that if these ideas are not so, than he is not a writer and he does not know what love is:
If this be error and upon me proved,
I never writ, nor no man ever loved
But this is a poetic device and what he means is that he is certain that he is correct.
There isn't anything that goes against Feminism in this sonnet. It elevates women -- unless you are talking about an extreme femininist view that men are trash and no one needs them, and who cares about love, it's all a male plot to subjugate women, blah blah blah.......