This is certainly situational irony because during that time period, it was just assumed that young girls were virgins. Why would anyone think anything else of a beautiful, young girl? But it wasn't the author that says this in the story, it is his character Angel Clare (see pg. 73 in etext). The reason this statement is ironic is because it will become an issue later after Angel marries Tess. It is this statement that reveals his assumption of Tess (and of most other religious girls of the day) that later breaks them apart. It is about this very subject of sexual purity that tears Tess apart from society with others as well as her decision to marry Angel. Angel has a choice later on down the road; and, that is whether or not he will forgive her past or hold it against her. That is why it is pivotal that the reader understands that Clares says this to himself when he first recognizes Tess at the dairy during breakfast one morning. Had Clare remembered his first impression of her when she confessed her sins to him, he may have been better able to forgive her and stay with her.
The above-mentioned comment of Hardy, about Tess, in your question, is true to the core in spite of its being a situational irony. Hardy's Tess is an innocent, down-to-earth girl of seventeen brimming with her fresh youth and beauty. But this pure beauty through which Nature manifests herself ignites lust quite surprisingly, in the minds of Alec D'Urberville, a church-person. As a consequence she gets robbed of her virginity in a dark moment. Thereafter when she starts her life afresh as a milkmaid Angel makes this comment being fascinated by her natural charms at his first sight. According to the situation of the plot this comment is ironical. But Hardy himself regards Tess as a Pure Woman in the light of the reason that being physically molested she does not have lost her intrinsic purity and virginity. Throughout the novel she remains a fresh and virginlal daughter of the mother Nature.