The two famous heroines are Helen of Troy and Isolde (Iseult). Helen was married to Menelaus, ran away with Paris and this was what started the Trojan war in The Illiad. Isolde was betrothed to King Mark of Cornwall, but fell in love with Tristan (Tristan and Iseult), a similar story to the love triangle of King Arthur, Guinevere and Lancelot.
Millay used these examples probably autobiographically. She was married to Eugene Boissevain and having an affair with George Dillon. It is supposed that Millay and Boissevain had a marriage which was open to outside sexual relationships. She (the speaker in the poem) compares herself to these women, in that, like them, she "suffers love" with the idea that her love/mixed feelings is just as grand as the love suffered by these legendary people.
But, since Millay was a feminist, there is probably a hint that the themes of forbidden love and affairs do not, or should not, carry the idea that the woman bears the majority of the blame. Also, along this idea, the phrase men use to refer to being stuck in marriage "ball and chain" could be comparable to, from the women's perspective "a ring." Both symbols traditionally meaning different things, but both having the same function: being tied to someone. Millay, like Helen and Isolde experience love outside marriage: forbidden and complicated but freeing as well.