Sonnet 124 is a beautiful sonnet about a higher love beyond the ordinary; it is a love that is above the common detractors or problems that come with most everyday loves. This particular love seems to have an ethereal quality, basically a soul mate type of love; the two lovers were born to love each other and nothing on earth can separate them from their love.
Sure. The entire poem is a kind of If/but/therefore statement. IF the poet's love were common, it would be vulnerable to common threats. Time would destroy it, the weather would affect it, human politics would affect it, etc. BUT it is not that way (he implies that it is somehow very special), so THEREFORE, it will last forever, unchanging.