OP - if, at any time, for any reason, you do not want to hear/see/witness other people having sex, don't go to the place where other people will be having sex.
It really is that simple.
If it was an issue about "tonight only", then stay home. Or go there and have them stay at your place. Get a hotel room. Go for a car ride. Pitch a tent in the back yard. You make it sound like you had no choice. Do you folks have one of those "rules" that you can only hang out or fool around when all 4 are present under the same roof? If so, then you should expect that there are going to be problems when one or more are up for it or down for it, and the other(s) are not. If you have that rule, and the purpose of it is supposed to be to keep things "equal" or to "avoid jealousy", well... "how's that workin' for ya?"
Another way of putting this is "don't go to the beach then bitch about all the sand." "Don't go to a rock concert and then complain about the loud music" "don't go to a bakery and complain that you can't eat the bread because you have that gluten-free disease" "don't go to Mexico and complain that there are too many Mexicans" etc.