"that the typical reader of LW is 20-34 and heterosexual and single and male and we prefer it that way." I think those effects are real - I would personally feel that LW was just a tiny bit less welcoming with such a post - even if I don't have a good sense of how to measure them.

This is a very good point, the disclaimer itself hammers home some information that may move the reader to a certain impression of what LW is.

In some ways I prefer not to know too much about the typical LW reader. Especially if I learned their political affiliations it might activate unwarranted biases against them.