It's the same with the party and the room. From my point of view, they don't respect your space at all. Your house, your room, whenever you aren't there they do what they please (throwing a party, re-decorating) and if you come back and see that things were changed, that things happened in your private space where you should feel in control and at home basically, and you get upset about it, they tell you to suck it up and don't see what they did.
There is no overreaction in your email you explained your thoughts calmly and not aggressively. Stand your ground, one part of feeling sad seems to be the violation of your boundaries, at least that's how I see it.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.