Legality tends to increase the participation rate in most things. As in the example from the resent case, adoption was bared or difficult before. Now it will be easier I assume.
The value of justice isn't something you can quantify as a numeric benefit IMO. Its an immutable good. So it doesn't matter if the number of gay people and their children being denied justice was 100 or 100,000+. Now society is more just and thats a benefit to all.
Or were you talking about the number of kids gays might raise as the good? I could see that action as quantifiable. It looks like this impacts more than 200k in 2012 which is about double what it was in 2000. This website has the following info:
http://www.lifelongadoptions.com/lgbt-adoption/lgbt-adoption-statistics
As reported on the 2000 Census, about 65,000 children lived with same sex parents. In 2012, 110,000 live with gay parents.
Households with Children in the US
Same-Sex Couples
Households with children 94,627
Biological only 72.80%
Step only or adopted only 21.20%
Combination 6%