I would like to point out that this phenomena exists for both gay men and women, its just that a lesbian couple produces less income than a heterosexual couple due to the gender wage gap; however a lesbian individual still produces more income on average than a heterosexual woman.

But with regards to your hypothesis, this seems very likely to me! I've never considered that homosexual couples are vastly less likely to have children. I find it interesting to note that even if the demands of the career choice are the same, a childless individual would likely still be able to produce more income at the same employer with the same hourly rate, as they likely have more time on their hands, and can therefore work more overtime. Moreover, being able to dedicate more time to work would likely result in faster promotions.