As soon as shipping became even remotely serious in the comic itself.
When it started with the Equius and Aradia stuff it was funny, and the context was weird and absurd enough that it fit in really well.
The love triangle with Sollux Feferi and Eridan stayed almost totally in the background til horrorstuck started, and I think that worked well.
KarkatxTerezi was a more serious thing but it had like one page.
I guess the serious shipping started more in 5.2. But in that act almost all the ships all had impact on the plot, and the build up to that impact was pretty tangible. We were all just waiting to see what disaster John and Vriska's interactions would lead to, and it ended up creating bec noir which was (at the time) a big deal. The cold war game Vriska and Terezi were playing with Dave and John also gave the story a certain amount of tension.
Looking back, the act 5 shipping wasnt that bad. There was Karkat x Terezi, but that ended up mostly being used to get Karkat pissed at Dave which was always pretty funny.
So I guess act 6 is when it got terrible with the shipping. The clusterfuck of teen drama with the alphas was just trash. And it bled into all other characters.