Why not make it like how the Darkside is? You can only be a member and its not searchable by google?
A couple of reasons that I can think of but they are more from an administration direction than anything else. Main forum rules, dark side rules or a third set of rules on the forum for people to have to deal with? Main forum rules might be a bit stilted in a singles section although it might keep the list of desired kinks in a partner down. Dark Side rules would be too open, a lot of our members won't participate in the Dark Side except for the Book section, and you'd get a lot more ummm... detail... than many might wish. A third set of rules just for that section would be cumbersome for the members and moderation.
The second reason is if you wanted it to truly be a 'singles' section, then a social group could be exclusive, at least exclude people who prove over time they are not single, just window shopping, or people who have some serious gripes against the opposite sex. In a social group situation, if the rules are laid out clearly to start with, violators could be removed from the group by either the group leader or a moderator if needed. In a main forum situation, trolling and flaming would be a nightmare to control and we have no way of keeping ineligible people from posting. It would be a moderating disaster area.
If you think it's a problem keeping the men from posting in the Ladies Section because that is reserved just for the ladies, keeping the trolling, ineligible people just looking for a weekend hookup because their wife or husband has gone to visit her mama would really be a nightmare.
Just my random thoughts on it. I'm not even against a singles' section, I was just trying to think of how to set it up where it would be the most open and friendly to the people who participate without being on display to the rest of the forum. Nothing I am suggesting is from a position of ability to make it happen, I was just exploring the idea of a social group because I've seen several large invitation only social groups be quite successful recently.
As a long time married person who knows she's too old to start over if it would ever be necessary, I don't have a clue about dating in this century, I just know a few of the ways to make the forum more usable to more people. {shrug}