Sissy doesn't value marriage very much. It is a societal requirement, something that comes as a consequence of her free-spirited, highly-sexed nature, and it is also something which she can manipulate to suit her convenience.
Sissy was first married, to a man of twenty-five, when she was fourteen. It was not her first romance, and it occurred because the man beat up her father when challenged "instead of the other way around". Sissy was married at City Hall, swearing that she was eighteen, not fourteen.
Sissy's first husband was a good man, and she "demanded little from him except a lot of love-making". By the time she was twenty, she had borne four children, all of them stillborn. Deciding that "nothing but death grew out of their love-making", Sissy asked her husband to leave her.
Sissy was married a second time a short while later. Because divorce "was complicated and expensive", she returned to City Hall and, "saying nothing about her previous marriage", was married again. Sissy reasoned illogically but conveniently that since she was a Catholic, she didn't believe in divorce, and that since she had not been married in the Church in the first place, it had not been a real marriage and shouldn't stand in the way of her marrying a second time. After giving birth to four more dead children, however, Sissy dissolved this marriage as well, telling her Protestant husband that "since the Catholic Church didn't recognize her marriage, she didn't recognize it either". Sissy eventually married a third time, again at City Hall (Chapter 7).