Hhmm I just started a romance with IB a second time (first time was a failure) and I just got the first sex scene with him. This is what I did.
After recruiting IB I went through all his dialogue options in Haven including the flirt options.
Took him with me on all quests just to help with approval during side quests in Hinterlands. Talk to Bron after building watchtowers with IB in your party, help people at the crossroads (guy who needs medicine for his wife, and the guy you give ram meat to) those got his approval for me also do the horse races.
Took him with me on In Hushed Whispers and conscripted the mages. Judge Alexius and behead him.
Went ahead and got Skyhold and met the chargers and went to Crestwood to meet Hawke with IB in my party asked Alistair if the calling controls the wardens (for some reason that got Bulls slight approval) also say the wardens are desperate (top dialogue choice) that also gains Bulls slight approval.
After that I went back to Skyhold and got Bulls companion quest and secured alliance with the Qunari afterwards apologized to Bull about losing his men. Went up on the battlements with him and flirted immediately after that I got the 1st sex scene (talk to him in tavern for it to start it'll just go into the scene)
Now I believe after the 1st sex scene you go talk to him again in the tavern and you'll get a heart option (we need to talk) or something like that.
However after the 1st sex scene I had to get off cuz my kids woke from their nap and I was unsure of the level of adult content I was about to see, so ill wait till later tonight and see if I'm able to get the heart option (we need to talk) which I'll post my results tonight

Hope this helps, I was able to trigger it without killing any dragons, in fact I was a level 8 when the romance officially started (yea I was rushing it

)