My first reaction is that this question should actually have been one of your first statements in your business plan for the site.

I think there's no question that a main photo should be clothed or cropped. For whatever reason - fed regulations or tastefulness.

On the inside you might was to distinguish between soft and erect explicit photos.

MySpace is boring without frontal nudes. ModelMayhem is full of frontal nudes, other than the main photo.

If you censor explicit, or put a fee on it, you probably are going to go down the drain. Which then takes you back to your business plan: "Why?" did you start the dating site?

Advertising supported? Click thru's to sponsor programs which you are an affiliate of? Member fees? Tangible sales of products?

If you're there to make money off of click thru's or other revenue, I'd say let the nude photos roll and make the site as fun as possible.

What's your defined competition? What do they do that you want to do, and don't want to do?