Okay. Monica and Chandler's relationship officially began almost 2 years ago at Ross's wedding? Correct? Okay. But the question of this coupling is when their relationship really began?
1. One instance would be in the episode TOW the flashback. We see the friends before they became friends. Towards the end of the show we see Chandler comforting Monica after finding out that Pheobe moved out of their apartment to move in with her grandmother because she couldn't stand Monica's clean pickiness. The hug lasted a while but then they ceased and continued with their ordinary lives. But, they felt something with that hug. Right?
2. Another instance. In the episodes TOW the beach and TOW the Jellyfish, Chandler was trying to pick up Monica. Insisting that he would be a great guy to go out with. He failed, unfortunately, but who knows if Monica was thinking about it.
3. And finally in the episode TOW all the free porn, in Chandler and Joey's old apartment(Monica and Rachael's new apartment) we see Monica and Chandler cuddling in the big chair. My reaction? What's this? They hadn't started going out yet, and yet, they are cuddling like a cute little couple. There was plenty of room to sit, but they managed to sit together in the big comfy chair? Again, were they thinking about each other at the time?
I find that these three examples are the producers telling us that this relationship was going to happen but weren't sure how or when until TOW ross's wedding came about.