I think you can interpret it two ways:
1) Katara accepted that if Aang carried out the Harmony Restoration momevent they would have to split up (???). And thus who doesn’t start a break up by making out?
2) Katara accepted that she and Aang were going to have to be apart for at least ten minutes. Cue tears and making out.
Personally I much prefer the second explanation cause it’s way more fun.