The short answer is housework and possibly even sex.

The long answer is from a new world-wide study published in the Journal of Family Issues which finds "married men do less housework then cohabitating men."  [live-in boyfriends]   Their "key finding" stated:

Couples with an egalitarian view on gender—seeing men and women as equal—are more likely to divide the household chores equally. However, in married relationships, even if an egalitarian viewpoint is present, men still report doing less housework than their wives.

In reading thoughout George Mason University's 27 page research paper there was nothing on at-home dad's division of labor except for this gem...

[This] leads to the question whether the relative-resources perspective—that places financial contributions in an exchange relationship with household hours—is in itself gendered. 

I didn't understand the last sentence either, luckily they knew we wouldnt either and translated to:

Are men more likely to see money as a way to “buy out” of housework, and women more likely to view money as power within the relationship that is not as directly tied to hours of housework?  

Reading this, I suddenly remembered a free-wheeling discussion on sex and the at-home dad headed by Brian Reid of Rebel Dad during the At-Home Dad Convention a few years back. During the discussion one mom sitting among the dads suddenly jumped up and said, "when my husband cleans up the house, and has that supper ready for her on the table that's all I need... I am ready for sex".  

Judging from the several other dads who nodded in silent agreement that day I think a better topic could be Effects of Union Type on Division of Household Labor and Sex.