It benefits the woman too.
Because her sons, carriers of her genes and the genes of her astute husband, will be more likely to produce and raise HER grandchildren, and not those of other men.
So it benefits the woman by enhancing her chances of passing her genes on down through the generatons.
After all, what woman is attracted to a sap, unless she already has children (or a pregnancy or eyes for the milkman on the side) in need of a father?
Which brings me to the alternate female genetic strategy:
Some other women would take a different reproductive strategy: get impregnated by "bad boys" and then find a sucker to marry and raise the "bad boy's" child.
This enhances the chances that any sons the woman may have, will impregnate other round-heeled women, who would raise her grandchildren however they can.
Now that is a good point I hadn't considered. As I understand your statement, a woman who is impregnated by a "bad boy", but who manages to raise sons successfully (with help of the "sucker", would see her genes carried to more grandchildren because of the "bad boy" genes. Hmmm. That is quite interesting.
Not being an evolutionary biologist, I can't speak knowledgeably about the representation of that strategy in observable nature, but I must say it makes sense.
(And of course, we're assuming that at lest some "bad boy" characteristics can be transmitted genetically, etc. etc.)