But there is a motherhood pay gap. Interesting research:
Studies from countries with laws against discrimination on the basis of sexual orientation suggest that gay and lesbian employees report more incidents of harassment and are more likely to report experiencing unfair treatment in the labor market than are heterosexual employees. Gay men are found to earn less than comparably skilled and experienced heterosexual men. For lesbians, the patterns are ambiguous: in some countries they have been found to earn less than their heterosexual counterparts, while in others they earn the same or more.
The results for the UK are that gay men earn less than hetero, lesbians more than hetero women. In fact, lesbians earn around what men do and gay men earn around what hetero women do.
We could, as this report does, speculate about societal standards, the idea that lesbian women have, in some manner, "male traits" which lead to that higher pay.
A much simpler observation of the evidence would be the influence of children. We know that fathers earn more than non-fathers among hetero men (yes, even after adjusting for age and education etc). Also that mothers earn less than non-mothers. Gay men tend not to be fathers (this is not being categorical of course, "tend") as lesbians tend not to be mothers.
If the so-called gender pay gap were simply the influence of children upon earning patterns, as we largely think it is, then we would expect to see what we do see when looking at the earnings of non-hetero society. This does not prove we are correct of course, but it is supportive of our view.