We currently have a member of our chapter who is married but she wasn't when she went through the recruitment process. She got married because of health reasons and it was easier to marry her boyfriend so she could live with him. He helps take care of her and the situation works better for her. Her health problems are pretty bad so she doesn't participate in a lot because of them so I think her situation might be different than most though.
If a girl wanted to devote time to the chapter and was an enthusiastic, active member of the chapter I don't think she should be denied the opportunity to join greek life. With that said if she is joining just for something else on her resume (as do many unmarried women do) they shouldn't seek membership.