>> Example, families with more than two parents (that is a real possibility due to overpopulation)"

For some reasons I thought Europe has an aging population issue and demographic decline not overpopulation.

>> families with more than two parents

I never heard of this unless you consider divorced people.

>> families with more than two parents

> I never heard of this unless you consider divorced people.

Look for articles concerning co-parenting, that's where its most often discussed. Wikipedia: "Co-parenting is an enterprise undertaken by two or more adults who together take on the socialization, care, and upbringing of children for whom they share equal responsibility[1] Co-parents may include a variety of configurations, including a mother and a father, two mothers, two fathers, a parent with an adult sibling or grandparent, or a parent and another adult relative. The co-parent relationship differs from an intimate relationship between adults in that it focuses solely on the child."

The other typical configuration would be in polyamorous relationships, when more than two of the partners decide to take responsibility of the child. Granted "typical" is a bit of a difficult word when we're talking about niche concepts. Most of my friends and coworkers have never heard about co-parenting or any other of the more diverse and sometimes complex family-alike structures, unless they concern failed relationships (see divorces). Which is kind of a bummer, that only failure in previous relationships motivates them to experiment with other concepts.

>> Most of my friends and coworkers have never heard about co-parenting or any other of the more diverse and sometimes complex family-alike structures.

I don't think it's worth to mangle obscure/exotic issues with mainstream subjects (overpopulation).

> "They accept that the structure of the family will change ? Example, families with more than two parents (that is a real possibility due to overpopulation)"

What does this mean? How is alleged population related to family structure and sexual relations?

I think grandparent was alluding to a separation of the two concepts of sexual relations and family structure: (Two or more) People can decide to bring up a child together without being in a romantic/sexual relationship. (Two or more) People can be in a romantic and/or sexual relationship without sharing responsibility of a child of one of the partners. Under this prerequisite, a family is any close social connection surrounding children (and the closeness and type of closeness being the linchpin of whether the term applies – not every flat share is a family for instance).

Regarding overpopulation: One motivation that I've heard quite often is that one of the most effective ways to reduce your carbon footprint is to have one child less. This is always presented with the asterisk that this is of course a very personal decision and there is the understanding that this is not a standard under which you will be measured. Still, if you're just looking at the numbers, that's what you get. Sharing parentage with more than two people is an alluring way to achieve the same result, especially as it means you will retain more of your personal time when child care is shouldered by three (or more) people.