This depends on what you mean by "online dating".
If you mean the "modern http-based web (aka InterTubes™)", that will take a bit of research to pin down in terms of "verifiably chronological ancestor to all dating websites"… Definite early front-runners as far as mass adoption would be Match.com and YourOneAndOnly.com, which Match later acquired and replaced its own offering with (1993-1994 timeframe).
Prior to that, however, numerous "sites" existed for online dating via USENET, Gopher, Telnet-based BBSes and MUDs, etc.
Even further back, proprietary dialup services like AOL, Compuserve, Prodigy and the like, as well as local dialup offerings in major metro areas running PC-based BBS software (only one I can personally recall is "Date-a-Del", running a Citadel variant in Minneapolis, MN, certainly active at least as far back as 1988) all offered various solutions to post and respond to personals advertisements, or even rely on computer matching algorithms to select potential mates.
And none of these includes all the sites/services that were used extensively for dating purposes as a by-product of some other function or intended mode of communication.
Note: I included dialup candidates in this answer specifically because of the origins of the word "online" itself in the context of computing – "a central computer establishing a connection to a peripheral or remote device".