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.
Given alcohol, poor hygiene, etc amongst outlaws and runaway serfs possibly an issue for goblin women too
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...
