A tip for entrepreneurs raising money: Don't make an investor wrong

I recently had a conversation with an entrepreneur about possibly investing in his company.  The entrepreneur asked for my opinion on the opportunity.  I told the entrepreneur I was interested in the business but wasn't willing to invest yet because he hadn't brought the cost of customer acquisition down enough (i.e. he needed to innovate on marketing). 
His reaction turned me off: he spent the next 5 minutes telling me why the cost of customer acquisition didn't materially matter and why I was wrong....or focused on the wrong thing. I appreciated his spirit of argument....but I really didn't want to argue with him.  (Differing opinions are welcome but argueing is a pain in the ass)
I wanted to interrupt him and tell him that my opinion was just that -- my opinion. It's neither right nor wrong....but whatever you do, don't fight with me and position me as wrong. I'll just retreat.  It's a delicate balance for an entrepreneur to make a convincing pitch in any environment and that balance has gotten harder to maintain in this environment. However, it's all the more important to make sure you guide your investors to coming to the right conclusion -- a big part of keeping that balance is making sure that you give the investor the space to have their opinion, to change their mind, and ultimately to make a choice of whether to invest or not.