Here are some options that make you situation safe with google and other search engines:
- If you have no script programming experience (For example, if you build websites using point-and-click operations in the wordpress interface instead of writing code in PHP to produce webpages), then your best bet is to make every user-submitted link have the no-follow attribute, and then manually have your moderators change the quality links so they no longer have the no-follow attribute.
- Create server code that scans the text contents of the webpages the submitted URL's point to and then have it check the retrieved text against your regulations and search engines regulations to determine if it is acceptable. If it is not, then either add the no-follow attribute to the link, or if the link is terrible, you could reject the link (have it not displayed).
For example, if your site is about fruits and a user submits a URL to a bicycle shop, then your script would add a no-follow attribute to the URL because the text in the URL never mentions fruits. just bikes. You can also setup your script to include filters so that if a user submits a URL about adult material, you can easily reject it, just because a word found in the webpage of the submitted URL matches a forbidden word (example: sex).
Do understand that if you don't use the no-follow attribute, then the links are assumed to be "follow"-able. This means google and other search engines assume they have permission to access the links that aren't marked "no-follow". For this reason, I'd recommend one of my two approaches, because if you allow link-following by default and you have no programming experience, but instead moderators, then guests could submit unacceptable material (for example, a link to sex material), and next thing you know, search engines could crawl that and possibly lower your ranking in search engines, just because your moderators won't have a chance to remove the bad link in time.