I agree with ysaye. from my understanding, the 'survival' in summer A's point of view is not the 'survival of the group' or 'survival of japan' it's just 'survival of self'
they are taught to abandon member of the group if they lower their chance of survival. As for the incest concern. believe it or not, it's not a big deal as you think it is. Egyptian dynasty have practiced incest for millenia between brother and a sister. suprise suprise, they didn't just die off because of their deformality, even after thousands of years. Beside, marriage between cousin was the norm even in darwin's days.
I do not think "having children with as many man as she can" is a good idea at all. When you settle into a new inviroment, there's limited amount of resources, as well as high risk of danger. Having a lot of vernerable children and pragnant women around will greatly decrease the change of survival. (producing children cost a LOT of energy) so women need to be picky about her mates and limit the number of offsprings.
Think about it this way. Why aren't all the members of team summer A having sex and raising children right now? why do they think that kurumi's pragnancy was too risky?