I am going to sleep with my mom's best friend in secret.
That seems weird. Why would you want to sleep with your mom's best friend?
I don't know why is this not good, but my body really wants her. She's like 40 years older than me, single and very attractive
It is wrong to sleep with your parent's friends. They are too old for you and it will become very weird and complicated.
It's alright. It is not complicated at all She has no boyfriend and I'm just curious.
But if it doesn't work out then your mom's friendship might be at risk. That is way it is bad to sleep with any friends of your mom because it will affect her relationship with them.