P.S. Wow. I've always thought that this is a very vibrant community and full of intelligent people. It seems I was right. But man even if I'd wish I just can't read all the comments and everything in here, and I'm sure there many worth reading.
In some birds and mammals, females will choose males that have a bigger or better territory. In some cases, if a male has an especially big territory, the male will attract more than one female. (Emlen S. & Oring, L. 1977. Science 197 , 215-222)
In insects, which tend to not have bi-parental care, this is usually in the form of a "nuptial gift" which is basically a nutritious package that comes along with the sperm (Vahed, K. 1998. The function of nuptial feeding in insects: A review of empirical studies. Biol. Rev, 73:43-78.)
You have the workload of a house divided into two instead of one. Even if your wife works it still ends up on the plus side.
It's not like you can code 24/7, you have to have some downtime. No reason not to spend that time with someone else.
* Incentive to earn more - esp if we're looking at averages, the $0 startup guys will drag down the singles category.. The married people will likely be after a paycheque of some sort
* Age. - There's usually a link between age and income - and i'd guess at a link between age & partner status..
* Just realised that the raw data is actually there - so I shouldn't be guessing... Haven't got time this second to go through it - I'll try later..