I don't truly have any answers about fear of having a romantic relationship. It does open a door to feeling vulnerable.
Are you in scenarios of social involvement, in person, where you are meeting people? Having casual chit chats? Making new acquaintances and potential friendships?
Knowing if you(plural/in general) want something further, also involves a sense of chemistry, which might not be evident, immediately or certainly unknown when just online.
And even if there's a sort of chemistry, in person, there could be other important facts where one needs to use their brain over their heart or chemistry, which takes time in gathering important information about compatibility.

Maybe those met from online lacked key pieces to the intimate equation?