Well, goblins vary in BR from the 3' stereotypical types up to hulking bugbears, so as Libor notes it's clearly an issue for human women.
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Given alcohol, poor hygiene, etc amongst outlaws and runaway serfs possibly an issue for goblin women too
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Half-sidhe are known in BR, half-Karamhul mentioned in some other D&D campaign settings, half-goblins not so much. In practice I suspect that relatively incompatible genotypes, infanticide, and the brutality and high infant mortality of goblin life would make the issue mainly a moot point - and frankly given the variation in goblins any half-breed would simply be a "goblin" in practice - maybe a "cute goblin" if they were lucky.
Goblins are raiders to most humans so outside of the 5-peaks (and maybe Thurazor) I'd say that they are unknown (maybe rumoured in a "if you fail to worship X your children will be born twisted..." kind of way), in Thurazor I'd expect them to be vanishingly rare, in the Peaks I'd say maybe - not convinced either way. I feel similarly about half-ogres which would be a 5-Peaks issue, but I draw the line at gnolls and trolls...
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