This is a very good question, it has made me think hard.
The emotion of love is hard to define. A lot of you have said the same thing in different ways in this thread.
Even the love I feel for my husband has changed over the years. But the love I feel for my children is unconditional, tough love sometimes and certainly sprinkled with pride and embarrassment at time. Whilst love for their father is less tolerant, I expect more from him and he can hurt me in ways that my children never can.
So I come down on the side of the love being different and of course we must consider when love stops with the father of our children. That happens more than ever now. But the love we feel for our children, even when they do their best to destroy it, overcomes everything and not to feel that might be considered 'unatural'.
Mothers who re-marry or take another partner I sense are in a real bind when a new partner is introduced into the family. Not being the father of the children with expectations that they should love their new partner children. When this is not the reality and they really don't love or even like the children they inherit, in my view the Mother should always, always protect and put her children first.
Then on the other hand, love for your children or not, the main issue is fairness. If there is an issue between natural father or new partners and children the main thing is to judge the situation and come down on the side of the wronged person, child or spouse or partner.
But sadly we live in a world where lots of children are living with strangers, their Mothers or even Fathers lovers. So whilst the question is simple "is it unnatural to love your partner MORE than your children" the answer is complex.
There really is no one answer, but being fair and honest will make everybody love you in the end.
It should also be said that the only people not related in a family are the Mother and the Father...think about that one!
The natural father as well as the mother will have love for their children, whilst a new partner cannot be expected to have that love, although I am sure it can and does happen.
Our children are always our children. Partners and even the father of our children can and do come and go. So you make your choice and take the consequences of what your children will think about your decision to choose someone else to love more than them when they are adults and you are old.