The single mother, is genetically and romantically worthless. She provides no value. She's been used up already and all the good experiences a man can have with her will be nullified by another man's presence through his children.

Women just don't want to face the reality of nature's chosen mating strategy for humans -- monogamy. When they break monogamy -- they become worthless. No amount of nagging or hagging brings value to the next male in line.