From the moment she entered the bath with Yuki, Agatha knew Ed was already banging Hiromi (which means Hiromi got a big headstart on Yuki, when it came to cheating). This probably added extra spice to Agatha's fling with Yuki.
I truly love seeing stuff like this. In most stories involving swingers, they...don't really show the two swingers loving one another. This couple, though, seems to genuinely, deeply love one another. Even when they have sex with other people, you can feel that they have total trust in one another, as if it was a perfectly natural thing.
And about the boyfriend and girlfriend cheating...Technically, it IS cheating, but I feel that they would end up being okay with it.
Overall, I just feel there were no hard or hurt feelings involved in this story, and I love that. I hate seeing the feelings of someone getting hurt. So, bravo on this manga!