I guess that this is why the children of single parents who are conceived through sperm/egg donation fare so poorly then, eh? /s
That'd be part of it.
There's a
lot of factors that go into successful or unsuccessful parenting, and what impact that has on the child. A couple of
really important ones that require having two parents in the home to have at all are:
1. Modelling commitment. The parents staying together demonstrates that commitments should be kept.
2. Being a role model for what a person of that gender should be like.
3. Modelling appropriate treatment of the other gender, especially in romance. A husband puts his wife's needs first, the wife puts the husband's needs first, is the ideal. Both respect each other's judgement, but can question it without insecurity causing a fight. That's the ideal, but even just getting some of this is better than not at all. One of the key reasons why single parenthood tends to be intergenerational, is because when the child becomes an adult, they have little or no idea what a healthy romance should look like in the first place, so they're trying to learn from scratch, or worse from popular media, most of which is worse than useless, filling people's heads with things that aren't just improbable, but flat-out
wrong.
Note that if a marriage is particularly toxic, then by how the couple
mistreat each other, it can actually be worse than a single parent upbringing, but you have to go pretty far down the hole to get that bad. Also note that single parents who actively work to overcome the limitations they and their children are suffering under can mitigate a lot of this, through things like having the children around grandparents with a healthy marriage, friends with a healthy marriage, making sure that a babysitter, teacher, pastor/youth pastor or similar is providing the other gender role in their children's lives, etc.
It's possible to do a decent job as a single parent, but the same behavior patterns that usually lead to single parenthood, usually also mean that the individual is very unlikely to do the
copious amounts of extra work to make up for the lack. And I haven't even mentioned all the practical disadvantages that most people are already aware of, such as only having one adult as caretaker and income provider, etc, etc.
The absolute best thing a single parent can do for their child, is to get married. Note that this is not a reasonable option for all individuals who find themselves in that place, but if they reasonably can, it's the best thing that they can do for the child or children.