Several asked for results of my informal survey.
It was about 50/50 on whether a boob is sexy if there is no nipple.
Some responses were emphatic that there needs to be a nipple
Some guys said there loved one has no nipple after a surgery and her breasts are still sexy. Big props to those guys.
Many responses expressed uncertainty on an answer.
Other responses recognized that seeing cleavage and side boob are exciting so it's more the boob itself.
Since straight guys don't see male nipples sexy, I do wonder if it is the boob itself that is what's truly sexy. However since the female nipple is entrenched in society as forbidden to be exposed in public, I believe that helps drive a guy to view the nipple as sexy.