If someone close to me were unable to have a child, and asked me to carry one for them, I'd seriously consider doing it for her. Maybe I would, maybe I wouldn't. But I would give it a lot of thoughtful discussion with Bob and other close family members.
However, if there was any hesitation on Bob's part that he wouldn't want me to do it, then I wouldn't.
It has nothing to do with his opinion taking priority over my opinion or the situation. It's just that carrying a child wouldn't just be an impact to me, but to US, and to the dynamics of OUR family. If me carrying someone else's child were to disrupt the harmony that we have, then I'd have to say "no", and focus instead on my own family.