I want to share something. Not to argue either side, but to share what I went through as a child ...and continue to go through as an adult.

My mother and father both cheated on each other. (multiple times)

My dad met my stepmother before he divorced my mother. They were separated, but still married. My mom and dad divorced (I think I was 2 or 3) and he then married my step mom. I believe I was 5 or 6.

My step mom was terrible to my dad's kids. She wasn't terrible in an obvious way. It was sometimes the way she'd look at me or talk to me....as simple as a face she would make. I knew, even as a small child, she wished I wasn't there.
She treated her own two children much nicer than us. My mom was talked bad about in front of me. Mostly about how unfit she was.

My step mom hinders my time with my dad greatly. And she still to this day treats her own children and now her own grandchildren with more kindness and respect.

I've worked really hard to develop a relationship with my step mom. Now we live close to them, so we are spending more time together. It's sad, but these issues still affect me profoundly.

Just yesterday I found myself sobbing over these feelings. Because in small excruciating ways she still behaves the same way.

I believe she should have never become a step mom. She should have never married my dad. But here I am an adult and I've accumulated years of baggage and self esteem issues.

I feel profoundly sad that I'll never have the relationship I want with my dad because of my step mom. My dad is a weak man and was a weak father. My brother and sisters all feel this way in one form or another.(minus maybe my oldest sister, as she blames my mom for a lot)

If you can be 100% positive that you could treat this child as your own and not harbor any ill feelings in regards to how he/she came about. By all means knock yourself out.
But you cannot, given the situation say that 100%

One day you'll have children of your own that you'll love more....let's just be honest. You might not be able to help not treating them different. This could possibly cause your step child lifelong scars.

As a betrayed wife, I know for me personally, every time I'd see that child I couldn't help but be triggered.

I ask you to search your soul on this one. Pray about it.