The question is about qualitative research (not tracking or usability testing).
Contextual inquiry (for example) is mostly described in the context of "work". Users are being observed while performing some tasks in their work environment (not at home). But even in this case many users feel uncomfortable being observed and asked question.
Online dating, however, is a very personal domain and users can mostly be observed at their homes. Would they feel comfortable to demonstrate their "real" behavior?
What do you think would be a creative way to observe their real behavior in online dating?