...Sir Walter's high-fallutin' talk on the subject of re-marriage is just his attempt to hide the fact that he has already been turned down twice when he proposed to women who were of much higher rank or fortune than himself!

As to why refusing to remarry should be considered a moral stand, JA's contemporaries would have known from experience that step-parents and step-children do not always get along, so Sir Walter is sparing his daughters any unpleasantness on that account!