In January 2012, The National Council of Family Relations released the report Family Formation and Raising Children Among  Same Sex Couples.  The report finds that the number of same sex couple with biological children from previous relationships and marriages is decreasing.  "Declining social stigma toward LGB people may mean that more are coming out earlier in life and are becoming less likely to have children with different-sex partners. Despite the proportional declines in parenting, analyses also show that adoptive parenting is clearly increasing."
You can read the full report at the link below.  If you want to dig deeper, check out the resources offered by The Williams Institute at UCLA.