I have recently registered the domain name spanish-fly.info to develop
a website as a means of capitalizing on the heavy traffic for this
search term. My question is: Will the major search engines
automatically treat this domain name as adult content, even though the
web site will be for general viewing?

Search engines will only treat it as adult content if they find adult
content on it.
If your spanish-fly website doesn't have anything adult on it and you
do not promote it as an adult website, then it'll be okay.

Just remember that, for traffic purposes, you'll be hurting
spanish-fly.info if you use it to promote your own line of paper
towels.  (Unless they are spanish fly brand.)