You can reconstruct what you want from your fit using the population fractions of male and female $f_M$ and $f_F = 1 - f_M$.

The expectation of $Y$ for males and females is given by: $$ E[Y|M] = a + b \qquad E[Y|F] = a$$ so $$ E[Y|M] - E[Y|F] = b$$ $b$ gives the expected change in $Y$ going from female to male, as you said in your post.

Given the male and female population fractions we can construct the unconditional expectation: $$E[Y] = f_F E[Y|F] + f_M E[Y|M] = (1-f_M) a + f_M (a + b) = a + f_M b$$ and from that, we can get the expected change in $Y$ going from sex-unknown to male: $$E[Y|M] - E[Y] = (1 - f_M) b$$ and, similarly, the expected change in $Y$ going from sex-unknown to female: $$E[Y|F] = E[Y] = - f_M b$$ You should get the same results with this method as from the procedure suggested by Michael above, without having to re-run the regression.

You can reconstruct what you want from your fit using the population fractions of male and female $f_M$ and $f_F = 1 - f_M$.

The expectation of $Y$ for males and females is given by: $$ E[Y|M] = a + b \qquad E[Y|F] = a$$ so $$ E[Y|M] - E[Y|F] = b$$ $b$ gives the expected change in $Y$ going from female to male, as you said in your post.

Given the male and female population fractions we can construct the unconditional expectation: $$E[Y] = f_F E[Y|F] + f_M E[Y|M] = (1-f_M) a + f_M (a + b) = a + f_M b$$ and from that, we can get the expected change in $Y$ going from sex-unknown to male: $$E[Y|M] - E[Y] = (1 - f_M) b$$ and, similarly, the expected change in $Y$ going from sex-unknown to female: $$E[Y|F] - E[Y] = - f_M b$$ You should get the same results with this method as from the procedure suggested by Michael above, without having to re-run the regression.