Agree with other comments here. You are not an asshole. Think on this- if it's a struggle to say no/be honest now, how difficult would it be later on if you two loved each other and she was pressuring you/feeling desperately sad because she doesn't have a child ? It's much easier and kinder to do it now, rather than a few years down the line.
It's completely ok(and necessary in my opinion) to be REALLY honest- Like don't be scared to use controversial sentences such as " Since I'm childfree, if the woman I was dating fell pregnant , I envision us having an abortion".
I know it's a hard thing to do. I always thought I was childfree , but I dated someone for a few years who had a child. He said he was completely ok with me not wanting any.
A few years in, he wanted more children. I still felt like I didn't really want any. But because I loved him and I wanted to make him happy ...I started to wonder (and hope?) If maybe I didn't really know my own mind , and that I would want them eventually , just as everyone says.
It wasn't the only reason we broke up, but it was certainly a contributing factor. We did love each other, but we just weren't compatible.
It's great that you are so sure of your own mind. It can take other people many messy years in relationships to realise that. Now you need to be confident enough to be uncompromising, on what you know is right for yourself.
I read once somewhere a study about "opposites attracting". The study uncovered that generally , the relationships that fared well , were the ones where the partners has similar attitudes/feelings about "the big things". Things like children, marriage etc.
I know that many people can date someone with opposite views on those things to themselves. But in my opinion and life experience- being with someone who has the same views on those things is just so much easier, less painful and less stressful.
This will be totally weird to bring in BDSM terms here. ( I know!) But - think of things like this as either a "hard" or a "soft" limit. Hard limits are things that you will flat out never do, and expect no pressuring/coaxing/manipulating in order to cross that line. A soft limit is something you may be willing to cross the line on.
I'm now in an amazing relationship with a fellow CF. I can't tell you the stark difference there is , when you are with someone who just "gets " this. We were friends for many years before dating , so we both already knew about our CF choice before we started dating. However, even us, who both have the same view on children -had to have the discussion about what happens if there was an accidental pregnancy.
My partner and I have been together for years now and I envision many more. However, should I ever find myself dating again - After my experiences I will never get involved with someone who either has children or isn't CF, because I know it would be bad for me.
So no you are not an asshole. Be honest both with yourself and your friend. I can't tell you what to decide , but I can tell you that in my experience being with a fellow CF partner is a much easier, less painful and less stressful than being with someone who isn't CF.
Sending you strength.