It's the ultimate irony that of all the people Touma's dad ran into in Academy City, he ran into Mikoto's mom.
And even "got a little on" with her there.
And then mom steps in almost Index-style, which makes it even more ironic since her inner self was Index back in the Angel Fall arc.
I wonder if Touya had his own harem when he was Touma's age.....
I have to admit though, I'd have loved to see that scene with both Touma's dad and Mikoto's mom being single.