Do you and S have agreements around talking to each other before becoming sexual with other people?
I don't think it's smart to get involved with people who are living with you without a lot of slowness and talking, especially with all the kids there who will be affected by fallout. ME in your place would insist that things stop until a lot of communication has happened. Obviously in your place I would be very upset with S if you had agreements, even if not. Fooling around with people outside of the house would be a problem for me without a heads up ahead of time. Fooling around with roommates without a heads up, that'd be a dead stop.
I'd be talking with the other two people about what had happened, how I felt about it, and ask them about their feelings. If L and B aren't able to speak clearly about all this (instead of the jokes about orgies and all), I'd be concerned they weren't mature enough to be attempting this while we were living together, and tell them I wasn't comfortable.
Now you may have very different feelings, other posters will have very different takes on this and they may be more useful for your particular relationshp, but that is just what I'd do in your situation. If S wont understand how rushing in isn't wise, and wont stop while you guys work out what is going on for each of you, well that sadly is another can of worms that you have there, so hopefully it wont come to that.
edit: I would also not feel trusting in this situation, as if they consider that you are an equal part of the relationship, why wouldn't S come upstairs and let you know what was going on, and ask you if you minded, or wanted to come be involved? Really, I would feel that it was sneaky whether they meant to be or not. It is behavior I would expect of people in their early twenties, not sure how old all of you are, but I would be disappointed. Are any of these things going on now similar to why your outside relationship and S's one outside fling had problems?
Happiness will never come to those who fail to appreciate what they already have.
Last edited by Anneintherain; 02-22-2012 at 08:48 PM.