Typically forcing a user to sign up / in before seeing anything would be a poor idea, but there is more to it in this case.
Dating sites can deal with a significant amounts of personal and perhaps sensitive information. Thus appearing "too open" with user data, allowing non-registered browsing or even search engine indexing, could scare off potential visitors. Requiring sign up creates a more "private" experience, which may make unsure visitors more comfortable to join.
On the other hand, dating sites get a bad reputation for faking large or unrealistic userbases to drive sign ups or payments (think of several Facebook adverts). Allowing a user to see actual profiles can promote trust in the legitimacy of the site. This is particularly important if the site takes payments and there is "something to lose", in which case trust is paramount.
So, this is a tradeoff, and strongly depends on what kind of site and community you are building.