Snape and Lily seemed very good friends and he was the one who told her she was magical, it seemed as if they were destined to be together yet they didn't end up that way. My theory is that they would have been, after all Lily was the only person who stood up for Snape, if Snape hadn't kept calling her names like mudblood and saying that he didn't need her. I think he actually really liked her and appreciated her help but was under pressure since Slytherins are considered to be the "pure-blood" house to say things like that to her and she probably decided that if he was going to do that to her then she wouldn't put up with him.

Snape most likely remained unmarried in the hope that Lily would change her mind and once she had died as a reminder of her and the job he needed to do to protect her son.