Judging from my social group, the gay male software engineers are more likely to be married than the straight ones (and I don’t think I could say that about any other profession), and also more likely to be married to another engineer. If I’m right about that, and the effect is strong enough, it could affect the numbers.

In my social group nearly all of the software developers I know are married to stay at home moms (self included). So it probably is just your social group.

You live in Seattle or the Bay Area perhaps?

I would suggest that in a field that is mostly male, including same sex couples changes the results disproportionately compared to more mixed industries.

Yes as long as SS couples keep it secret. Anonymous surveys are a great way to say that you have a "committed partner" without going into details.

Even with people keeping their relationships secret, I doubt SS numbers are that high in the software industry.