Could be a number of things, my theory is that they want to find a compatible race in terms of looks and breed off the population like say earth which hosts billions, their would be a large boom in numbers rather than the few women viltrumites forced to have multiple kids one at a time
Could be a number of things, my theory is that they want to find a compatible race in terms of looks and breed off the population like say earth which hosts billions, their would be a large boom in numbers rather than the few women viltrumites forced to have multiple kids one at a time