Many, maybe most, step parents are excellent parents and that could be a much better situation for the kids than a single parent.
But, not taking anything away from the responsible, loving step parent, the statistics show that there is a higher risk for children for abuse, especially sexual abuse, from a step parent than from a biological parent. There is also far less stress and more emotionally stability and reinforcement for kids when mom and dad love each other and live together than when their loyalties are divided between separate families.
We need to return to a culture in which marriage before kids is the norm and staying together as a couple is much more common than divorce.