> Will people who were genetically predisposed towards being thin, hard working, open minded to approaching members of opposite sex and forming deep attachments resulting in children become more frequent?

These are not the people right now having more children - it's those who (often at least partly for reasons of disadvantage) have relatively less impulse control and are poorer at long term thinking/planning.

And/or those who are part of a culture that encourages fertility and discourages contraception.

I don't see how current trends point to this changing in the direction you mention. Being young has more to do with thinness than genetics, and most babies are born outside of marriage.

Edit: added qualifications and clarifications