Putting your dating profile on a dating site, with preferences for matches, means explicitly "I want these specific people to look at my dating profile - FOR DATING". That is the obvious purpose of the site and the profile, and the "preferred matches" only reinforce that, not to mention the internal filters OkC allows you to use to block people who don't match a certain %.
There is not any explicit purpose or intent that says "I want this personal information to be used by researchers in a large group study". There was no explicit agreement between the user and these researchers. Hence, no consent.
However. The question then becomes, "Do they need consent?"
The answer is: Check the Terms Of Service on the website. If the website explicitly forbids using the data for research purposes, they can probably be sued by the website for breach of TOS. If the TOS does not explicitly prohibit research use, then no, they don't need consent.
Oh - and if it wasn't already obvious, the users don't own the data, so they have no say in how it is used.