I feel that "friend love" and "romantic love" are different. And I don't have children, but the kind of "protective attachment" I have for other people's children or for my cats is also completely different.
I think you can have friendship + lust (fuckbuddies) or romantic love + lust (traditional relationships) or friendship without lust (regular friendship) or romantic love without lust (platonic love).
I think "protective attachment" is not usually associated with lust because it's too likely to cause harm and therefore conflicts too much. I mean, from a psychological point of view, I think that's why, because there is a hierarchy there, while the other two are with people on the same level. The protective one is about devoting yourself for another being who needs you to survive, and therefore you stay focused on that.
Having lust for more than one person doesn't seem to be a problem for most people. It seems that mono people simply aren't wired for romantic love towards more than one person at a time, while the other types aren't limited. Poly people don't have limits on either.
I would compare with loving more than one friend rather than compare with loving more than one child, partly because I don't have children on my own and don't feel comfortable using an example I can't completely relate to, and partly because people will be less freaked out by the implication of having sex with friends.
I think most people can tell the difference between a friend you have sex with and a partner you are in love with. They are different feelings for sure. Love isn't friendship + sex, there is something else there.
So I find it normal that they would stimulate the brain in different ways, yes.