Babysitting & TTC

So my fiancé and I have been TTC for 2 years now and today he asked me if we could babysit his friend's 3 yr old tonight. So many thoughts and emotions came over me. I really don't want to (I've never met this child before) but I told him yes we can it's fine. I know what's really going to get to is seeing how good my fiancé is with kids it breaks me every time. I just have to not let my emotions get to me. Has anyone else been in a similar situation? Or know how I should handle it?