I do not think that is really what Rowling was implying with that statement. The full quote is as follows:
Nithya: Lily detested mulciber,averyif[sic] snape really loved her,why
didnt he sacrifice their company for her sake
J.K. Rowling: Well, that is Snape’s tragedy. Given his time over again
he would not have become a Death Eater, but like many insecure,
vulnerable people (like Wormtail) he craved membership of something
big and powerful, something impressive.
J.K. Rowling: He wanted Lily and he wanted Mulciber too. He never
really understood Lily’s aversion; he was so blinded by his attraction
to the dark side he thought she would find him impressive if he became
a real Death Eater.
From the context of the quote, it seems to be saying that Snape wanted what Lily could offer, but he also wanted what Mulciber (and the other Death Eaters) could offer; love and power respectively.
Consider Snape's life and perspectives as a boy/teen/young man. At that age, most guys simply have little concept of the long term consequences of their actions. One theme of Rowling's work is unconditional love. We see it in Lily's sacrifice for Harry, we see it in Harry's sacrifice for his friends. There's a big difference between love and desire. Snape desired Lily, but did he truly love her yet? He loved her presence and her kindness, certainly, but was he truly willing to live his life with and for her instead of just for himself? I don't think so. I think that's why Rowling calls it Snape's greatest tragedy. He chose the Death Eaters' companionship for what they could give him: power. All the people who mocked "Sniveling Snape" would certainly fear Snape the Death Eater. Snape had a clear choice to make: he could love Lily, and live his life with her in peace and quiet, or he could choose the Death Eaters, and finally end his years of torture at the hands of his enemies. As a child who grew up in a loveless world, I don't think Snape truly understood love. He understood fear and power far more. He thought he could choose power, and Lily would be drawn to him because of it. I think Snape began to really understand love as he saw Voldemort's hatred and evil unfold, at exact odds with the Lily he had feelings for from boyhood. When he found out that Lily was a target of Voldemort, it was then that Snape chose to love her more than his own position. He knew he would be largely hated and mistrusted by both Death Eaters and most of the Order from then on, but if by doing so he could protect Lily, then so be it.
We are not told if Snape had a relationship with Mulciber in any way other than as friends and coworkers under Voldemort, but we are told that Snape never stopped having feelings for Lily. His Patronus, an expression of his innermost being, reflected her. After she died, he dedicated his whole life to her memory. When considering the full context of that quote, and comparing what we are clearly told of Snape's life, I don't think we find any canonical support for a Snape/Mulciber relationship.
