Tess of the D'Urbervilles asks the reader to think about the fact that marriage is a social convention: it's a practice that was invented by people. It's something that we learn from society, but not an idea that we're born with. So what's the difference between social marriage, and natural marriage? What are the responsibilities and obligations of each kind of marriage? And who is Tess's real, "natural" husband?
Although Hardy frequently privileges the laws of nature over the laws of society, it is clear that Alec's claim that Tess is his "natural wife" should be rejected out of hand.
Despite having two "husbands" at different points in the novel, Tess is never actually married to either of them in the fullest sense of marriage that Hardy proposes.