Would it be possible to do it in two, more indirect, steps?
First, that you sit down and talk with them about how each of you feel when the other two are having sex, and you, for some reason, can not be involved? When they know precisely how you feel about it, they have got a signal. If you feel pretty sure they already know, you could still bring it up, just updating.
Next, you can simply tell them when you will probably be back again Thursday night.
Then, they should be able to add two and two correctly, shouldn't they?
And if it turns out they couldn't after all, you have a subject for a dialogue on Friday or later.
I think it is best to let the others know how I feel and react, and in the first place leave it to them to find ways of adapting to it.