Your take on the Emperor is solid, using sex as a manipulation tactic is as old as any of the races I am sure. The waiting for development for sex is two fold. These are companions you have to trust with your life. Has every sexual encounter you ever experienced ended well? Would you risk with no real relationship putting your life or the life of the party in jeopardy because a lover that feel betrayed?
Perhaps they should add that to the game, a chance to piss off a lover and have them half way through a fight turn on you and go psycho

I wanted to reply to this because it makes no sense. The two characters (and I guess Minthara) who propose you for sex very early on, make it quite clear that they are in it for the entertainment alone. Why should they feel especially betrayed or why should the sex add an extra layer of danger to the already perilous situation of sleeping around a campfire with a bunch of strangers? The dangers of which are illustrated through a whole bunch of late night encounters. In a few of them, the Durge-PC is even the aggressor - the Act 2 Durge scene is exactly what you were asking for, a lover turning on the other or you turning on your lover.
I am way more interested in how the relationships are treated and portrayed in general and in this regard, I am perfectly fine with Astarion, Gale and Lae'zel, but absolutely not with Karlach, Shart and Wyll. Astarion is trying to manipulate the PC at the start, but the game is very open about the manipulation. You get to make a perception check and Astarion uses a completely different voice when he is in performance mode than when he is sincere. The situation is a bit iffy and the game clearly tells you that it is. Gale is a sweetheart who first asks you if you are generally interested in "other forms of stimulation", then sets up a date and if you are not interested in his visions of the Weave, he'll switch to the magical four poster in the woods - like someone might who care's about the other person's feelings.
Karlach, Shart and Wyll on the other hand have a very fixed perception of what sort of romance they want and you better play your designated part in it. You can't just have a bottle of wine as friends with Shart or dance for the fun of it with Wyll, they will be disappointed if you don't treat these encounters as romantic interactions - and Karlach, well she is open about wanting to "suck face" with someone and you are just the person to work on her romantic bucket list with. I don't think these aren exactly great examples of relationships, yet the game treats them as perfectly normal and romantic. And if I should complain at all (I mean a lot of people seem to like these romances) my issues would be with this and not with in which Act someone wants to do the deed.