Antonia Fraser's "The Weaker Vessel: A Woman's Lot in 17th-Century England" mentions that it was a popular ideal, especially among the upper classes where money was involved, for a widow to remain faithful to her husband's memory, living in retirement and devoting the rest of her life to the upbringing and education of her children. (Women certainly did remarry, however, and there was hardly any trouble about widowers remarrying!).

Some very peculiar attitudes can be detected towards those widows who did take a second husband. There was an idea that marrying a widow might constitute "bigamy", and if a widow had two spouses dead, it might even be termed "trigamy"..."He that marries a widow makes himself Cuckold" (that is, by the woman's dead husband). (p.90)

Personally I think this was due to the good old double standard where sexual fidelity was expected of a woman, even beyond death, but not from a man, and I suppose if a woman remarried and had more children, the first husband's family fortune might not go strictly to his own children, but become "diluted" along the way.