You have to take on a lot of responsibility when you date somebody who has kids. You may not be responsible for their daily care or take on a parental role, but you have a duty to those children not to be a long-term negative disruption in their lives. It’s natural for a child to resent their parent having a boyfriend or girlfriend initially, but if they continue to resent them for years, there might be something wrong with the relationship itself. 

A relationship with a parent will not work if the partner who doesn’t have kids resents their partner’s kids and their responsibility for those kids. It’s natural for a partner to feel some level of sadness that they’re not the #1 priority in their partner’s life, but a little sadness doesn’t make or break a relationship. Long-term resentment absolutely will, and no kid deserves that kind of treatment.