Bob is a heavy sleeper and Linda knows this, so getting freaky with her son right then and there when Bob is sleeping is a calculated risk, it's the same risk you take when you have sex in public. So Linda is a little bit of an exhibitionist, that's great, I am too, it's great
And if Bob was to wake up all of a sudden, MC can quickly hide or some shit and Linda would have an excuse for being loud ready and Bob would think nothing of it. It's not like she WANTS Bob to see the two getting freaky, but just the risk of it is thrilling. I love how she thinks xD