Yes. Of course if you have multiple children they're individually less special to you!

Hmm... perhaps we don't mean the same thing when we use the word "special". If I pretend that you used a word unfamiliar to me instead and had to work only on context, where you continue with:

You have less time and energy for each, less brain-space to store facts about each

...then I'd have to agree with you. Certainly, I have less time and energy to devote to each child.

and you aren't even culturally allowed to have a favorite!

For the record, I never claimed to love them all equally, or to not have a favorite. (They are all my favorites, in different realms, but even so... it would be absurd to claim that it just happens to all add up to be equal.)

But I don't see what point you are making here. My point is that my love for the first child was not diminished by the arrival of the second. For some other definition of special (importance in my life), I would say that the first is just as special to me.

The reason this is brought up (perhaps mostly by poly people with more than one child) is that one's capacity for love, for this "specialness" is not fixed! Another child comes along, and your capacity grows. Another long-term, committed partner, and your capacity grows.

That is the point of the argument: capacity is not fixed in size.

As for why it should be different for lovers, the psychology about lovers and children is very different.

Certainly, but the point about specialness-capacity-increase is fairly general. I would apply it to lovers, to children, to favorite movies, to desserts, to symphonies... the more things we love (or are special or meaningful to us), the more our capacity increases. These things, these experiences make us grow. (Well, maybe not desserts; that's a different kind of growth.)

And we accept that this is how we work in terms of children, movies, food, music... why make an exception for lovers?

There is no reason in principle why we couldn't have been hardwired for extreme strict romantic monogamy and still love lots of children.

Ok. I suppose not. I suppose we could have been hardwired for extreme preference for only one flavor of ice-cream... Do you just really not like the comparisons between different categories of things we like/love/enjoy? Of course our feelings for these different categories are all very, very different, but the generalization seems valid enough to me.

And especially: if they feel similar enough to me for the generalization to hold, then I'm really not going to be convinced that I must love only one by the argument "romantic love is different because it's different". (Which isn't what you were saying, but it's the message this line of argument addresses.)