Why do people expect partners to abandon children of previous relationships ?

Because they never knew about fatherhood & motherhood,,, If they ever had real fathers or real mothers they could never have asked that,,, Or better say it in other way,,, They are not interested in their partners but their partners' bodies,,, And bodies come with no responsibilities,,,, Or do they have hearts of stones?

You know I don't think it has anything to do with the kids, I believe when this happens it is the jealousy of the one partner that their spouse has to share some of his life with someone in the previous relationship. They are not adult or mature enough or sure enough of their relationship to allow him/her to go and visit with the kids, even picking them up presents a problem because they have to see the ex. And many times him/her is persuaded to just let the kids go to keep the new relationship. Divorceis bad, but when small children don't get to see their dad, it leads them to believe they did something wrong, and it extends into the rest of his life.