well the first child will be a girl. but will bring a echidna and other monsters girls only female children to world? i dont think this is true but when it is true then this must be a curse to them, never be a partner of their own kind must be hard.
when a monstergirl choose a human man as her husband and he will the monstergirl as his wife too but when the man is widowed and have children from his former marriege with a human women will monstergirls accept this man in that case too? perhaps this possible not any men is single.
i think many human man are married with a human woman and a monstergirl and in that case a lot of human children have monstergirl half sisters. that monstergirls sometimes a step mother for human children it´s not unusually in this world but it would be more different when a widoed man have monstergirl children from a former marriege with a other monstergirl because some monster races hate each other so it´s easier for them to deal with human children.
if i depend on the monstergirls profile there are many races and types that have no problems to share a married human man with a human woman or accept children from a widoed man. the 3 mermaid types and the harpyies types are very close to this. the dwarf, sahuagin, dragon, yuki-onna, gigant ant, lizardman, skeleton, leanan sidhe, jurougumo, inari, cyclops, holstaurus, wererabbit, werecat, honeybee and golem looks to this factor too.
intersting is by the echidna profile that the word jealous not appear in there it´s seems she have no problems when her man meet other woman or can live esay with the fact share him with a woman if it´s ending in the relationship between a man, woman and monstergirl.
