I'm going to try to set aside my dislike of SSM and consider the question.

According to various credible sources, LGBT persons typically account for 3-7% of the general population. How many of them would be intrested in SSM? How many of those will be intrested in adopting or otherwise having children?

I don't know the answers to those questions, but I assume each subset is smaller than the previous subset.

Let's say LGBT's are 5% of the population. Some of them are bi and married to the opposite sex already. Some of them aren't intrested in marriage at all.

Let's look at Massachusetts, which legalized SSM in 2004.

Some facts...

Same-sex marriage in Massachusetts - Wikipedia, the free encyclopedia
If that rate held true since 2005, then there should be currently a total of 17,600 SSM couples in Massachusetts today, roughly. 64% were lesbians, and 40% of that subset had children in the home... I have no stats on male homosexual couples having children, for Massachusetts.

If we assume 3.5 persons per household (a reasonable median figure), Mass should have about

1,885,000 households. If 17,600 of them are SSM households, that is 0.9%.

We know that 64% are lesbian households, and that 40% of those have children involved. That's 0.2% of total households that we know are raising children, the traditional and societal purpose of "family".

0.2%

Even if we consider this a positive in itself, it obviously isn't going to have

*much* impact on the societal need for families responsibly raising children, it would seem.