An engaged co-parent (this is not just a few nice dinners out or an adventure buddy) has to do something along the lines of the following:
- Spend 1-2 hours a day with the kids
- Spend 4-8 hours on weekend kid duty
- Sick care/emergency care duties
- Holidays
- Maybe assisting with splitting time with the shared custody parent
For simplicity of argument, let’s consider that person that the co-parent is dating, only has the 50% custody, and so all time investment costs are halved.
This brings the time spent to 9-18 hours/week with the kids present, which means 4.5-9 hours per week in a 50/50 custody arrangement – plus the irregular duties. Considering the matter occupationally, a 10 hour/week side gig is $150 at a living wage, plus the fact that this is irregular duty pay, which brings some surcharge. Therefore, we’re likely talking about a retainer on the order of $200 a week. To the point of “common” – we aren’t talking about disabled/pensioners who just want some companionship; we’re talking about the mass of people, who have to work for a living. 10 hours a week/$200 isn’t going to get it done, so you have to consider that this person also has another full-time job. Even at minimum wage, that other job is going to pay $600/week, and if this person has the emotional stability/will to be raising kids in the off hours, they easily can get education (with that same off-work time) to make far more than $15/hour. So, from an economic perspective, this is not optimal for most people.
The next point would be that you want a certain lifestyle; for example, you want to live with this single parent, and the kids are part of the deal. However, if romance/sex isn’t involved, there are any number of people you could live with/spend evening time with that share common interests, that don’t require you to have screaming kids and drama. Moreover, single rooms can be had on the order of $600/month (25% of that minimum wage paycheck), which is going to give a lot more peace and quiet for studying (the above economic purpose) than a house full of kids.
Consider that you want the lifestyle of taking care of kids. Men and women always have the option of raising someone else’s as a job of some sort, or as a family relationship. If you like it that much, to accept such economic opportunity costs, why would you want to do it 50% of the time? Why would you want to only be involved as maybe a 25% parent, instead of 50% (your own kids, adoptees) or 100% (single parenting, child care as a profession)? You put forward a significant amount of inefficient effort, to yield an even less likely amount of future benefit than in an intact family (because kids aren’t obligated to help you e.g. when you are old). It’s not that the desire doesn’t exist or is invalid, but rather that there are far more personally beneficial ways to scratch the itch of raising children.
You also have to consider that the single parent (vs. someone without kids at home) is likely to have baggage, including that of the co-parent with whom the two of you must deal, and all the problems either of them have relating to relationships, and all the tension, threats, and general state of conflict. To mitigate those risks, you cannot accept or engage in promiscuity; Dana Bailey and Unique Harris would tell you about the danger, but they’re dead.
Moreover, finances and living situation flexibility are more limited.
Hence, entering into one of these relationships as an outsider (vs. e.g. meeting this person at work and growing closer together), is economically incorrect, lifestyle incorrect, and romantic relationship dicey, even if the person is serially monogamous. In some cases, we also must consider the possibilities of pregnancy, and if (as is highly likely in marriages to single parents) the romantic relationship ends, the previously childless co-parent is even worse off on all three dimensions. It’s accepting far more cost and taking more risk for less reward than the alternatives.
Since the point would be raised, “but you can get sex/love out of this relationship, can’t get that out of being a daycare worker”, for clarity, I think we directly should address this subset of the previous arguments. With the additional income you make from economically rational behavior (by working 50+ hours a week including off hours), you have the means to pay a lot of prostitutes. If you consider that you pay $400 dollars every week for sex, that’s still ~$21,000/year, which easily can be derived from a six figure income. Remember also that the sex you directly pay for is whatever you want it to be; you don’t have to be nice, gentle, etc. The sex you get from your single parent therefore has to be significantly better, and the obvious way to make it better is that, in addition to high frequency, it doesn’t involve condoms.