In approximately 15% of homosexual men, their phenotype is associated to the fraternal birth order. Older biological brothers induce in their mothers anti-male factors (antibodies) that interfere the brain maleness development of younger fetuses. This effect is seldom seen in non-right-handed men and is not seen in women. The influence of older siblings is seen in their sex ratio (SR). In contradiction with previous hypothesis, significant heterogeneities of SR have been found among older siblings of males or females, right or non-right-handed and homo or heterosexual individuals. This can only be understood as if the findings among homosexuals were part of a general mechanism of fetus-maternal tolerance-rejection processes of placental mammals. We found, in relation to ABO and Rh systems and sex, that embryos with genes different from those of their mothers, induced better pregnancies and maternal tolerance than embryos similar to their mothers. Assuming that homo or heterosexuality and right or non-right-handedness behave similar to ABO or Rh alleles, the author provides a speculative interpretation of these results. Homosexual women and especially if they are non-right-handed, are preceded by siblings with a high SR (maternal environment with anti-female or pro-male factors); then lesbianism or non-right-handedness may induce tolerance to be a woman in such anti-female environment. Non-right-handedness could induce tolerance for anti-male factors of mothers, thus preventing the production of gays in a pro-male maternal environment, but leading to the production of non-right-handed gays in anti-male maternal environments. Several new hypotheses and interpretations merge from this new proposition. Also, complete sexual orientation could be acquired after birth.