Why is it a common insult for someone to say they slept with your mom?

Well, once one in an adult, it becomes a bit silly for sure. But when we're kids, our parents are often the center of, if not the entirety of, our universe.

Since it's something attacking a fundamental building block of our identity (with the obvious caveat that there's exceptions to almost everything), and allowing someone to attack that is not something you can just shrug off when you're young, it becomes something bigger than its real import.

That often lingers into adulthood, where an attack on family is an attack on the self, as a matter of principle. If you allow those attacks to stand without any response, it can be taken as permission to do the same, or worse, again.

As such, anyone wanting to get a rise out of people can typically rely on mom disses to get the job done.

Your mom's sexuality isn't the core of the attack. That's just the window dressing to the real purpose, which is saying "you are lesser than me, and can not retaliate effectively when I attempt dominance". In the case of OP, as everyone knows, their mom is a total slag to begin with. That is regardless of who OP is.

As far as sleeping with someone older: doesn't matter; had sex. This is a simple rule of life, that if one is the sort to brag about sex at all, any and all sex is worth bragging about. To wit, I bang your mum then bang your drum. Which is to say that yo mama such a ho that her vag emits hawking radiation.

In other words, what's wrong with intercourse between two consenting adults of any age? While it is true that yo mama's pussy is so old that she queef mummy dust, it does not invalidate her value as a person or sexual partner. Moreover, just because yo mama so ugly tying a pork chop around her neck couldn't get dogs to play with her, does not mean she is not worthy of a good, deep dicking.

To the contrary, women of age enough to have a child old enough to ask this very question on a forum such as this likely have had a great deal of sex, and thus is an excellent choice of partner for the discerning gentleman, lady, or other. To rephrase, yo mama such a ho that they call her Chicken Coop because cocks be going in and out all day, so she know the good tricks, and she is so old the Flintstones moved out of her vagina to a better cave where they met the Rubbles.

There is no shame in your mother. Consider: she is the first person to be in both the NFL and NBA halls of fame for most balls taken to the face. Indeed, she is considered to be the greatest wide receiver ever to take on an entire team by herself. Sadly, she could not qualify as a tight end, as she is such a ho that they used to be called jumpolines before she got on one. Her name on the halls of fame plaques is Peanutbutter because she spreads so easy.

Besides, take pride that you have earned the envy of others by being such a wonderful person that they can't throw disses at you directly, and have to throw them at your mom the way I throw her the D every night.

Believe it or not, there was a serious answer in there.