If you think it is lewd or inappropriate for your website then it is already spam. Suspend or ban them for spam. They are only there (in my opinion) to get other people to click on their "favorite site" to generate money for them. ....or maybe they are just lewd and inappropriate...
My community is over 5430 members. The site rules are very specific and covers this area. We don't allow links to other sites. A member must have written permission from me or "admin" to post a link.
One day I had a link on the front page blog from a brand new member to a graphic porn site!
I'm pretty clear in the rules and TOS, so when there's a violation,I hit delete and move on. No worries about Political correctness, or any dialogue required. Just hit delete. No one has ever come back asking questions. Lots of people are really looking to serve their own interests with outside links. The focus should stay on the community.
Well, if you're asking adults what they're favorite websites are, and you want them to be honest, that's what you're going to get. I don't know if these people are spammers as James implies. Who knows?
Once upon a time, I did that myself. LOL Dude I knew set up his own little social network and invited me. One of the questions was what were my favorite sites. One of the sites I listed was an adult site. I wasn't trying to be crude, and it's not like his site was family or religion oriented (it was just a theme-less social site with a goofy name). Not that it would have made a difference, but back then I didn't use my real name and all my profile pics were of animals or pottery.