In temporal or conditional clauses in English (those introduced by words like when or if) we mark past meaning, but not future. So
When I saw him
is clearly past, but
When I see him
could be present or future. (The only kind of present it could be is a habitual, as in
When I see him, we always talk for too long.)
Is never idiomatic in English, and is arguably not grammatical.
So when the temporal clause is non-past, it is the main clause that marks whether it is to be taken as future or not.
So in the case you are asking about,
When I fall in love, it will be forever.
the main clause is marked as future, so the temporal clause must also be future, i.e. your meaning 2.
To get your meaning 1, the temporal clause would have to be past:
When I fell in love, it was forever
When I fell in love, it would be forever
(using "would" as a "future in the past").