Of course I was surprised and they were surprised to see me standing there. Then they both grinned modestly and a bit self-consciously, and they both laughed.
My mom explained that they weren't being lesbians, just two friends having a fling. I wasn't expecting this, but they were both straightforward about it, and I just said how that was fine with me, and I discretely left them alone.
After that whenever my mom's friend came over, they were quite relaxed about being a little bit affectionate around me, and they would kiss. Nothing passionate or anything like that, but just in a friendly way. I got used to see that and I was perfectly all right with it.
My mom also became more relaxed with being uninhibited around me, and I started seeing her naked a little more often, and I let her see me naked. It was really neat feeling that comfortable with each other.