I think a better question would be:

How common is it for secular humanists to take a stance against access to abortion (and/or) equal rights under the law for people who identify as something other than heterosexual?

I think the atheist part of the question is a bit strange since it really only informs us about whether or not a person has a belief in a god or deity, where as secular humanism tells us a bit more (in theory) about the social ideology of an individual.

I personally hate equivocating atheism with anything else other than the god/supernatural/intelligent design question because it can further justify the slippery slope of lumping other topics in with atheism as though it is a prescriptive ideology like religion tends to be. However, I understand what you are going for by asking this question, and I don’t want to be overly pedantic.

To answer your question:

I think, as another poster already said, it probably matters if we are talking about people who self-identify as atheists vs. people who don’t believe in god/gods but don’t spend a lot of time thinking about their beliefs.

Generally I would venture to guess that in the first group there is a large majority who also self-identify as secular humanists, and probably take a more progressive stance on access to abortion as well as equal rights for all people regardless of sexuality/gender identity. I’m not sure I have ever met a self-identified secular humanist who took an oppositional position on either issue.

As far as people who do not self-identify as secular humanist, I would say it probably mirrors the support for the issues in a relatively similar way as the general population in their geographical area.