Hi, I've read about this in another thread on another site, I think, and someone discovered the reason it was happening. Basically when traveling, if one of the foreigners is going to pass away soon and needs to "procreate" in order to populate the town, it will pick the sim with the highest relationship, and create a child. It does not have to be a romantic relationship. Usually it should do this with other foreigner townies, but as you can see its not always the case.
If you look at the other parent of the kid, it should be someone from the place you visited, and probably someone your Sim knew or met at one point.
I think married Sims are exempt because they are already in a relationship and not open to help repopulate the town that way.
So yes, traveling single will be a bit more dangerous!