This sentence was an old dead meme used for no absolute reason, but due to the fact that your mom being gay is true since most females are attracted to males. This sentence was originally used to insult someone and has became a meme and so did the no u meme.
The kind of argument usually starts off like this.
by far the most devastating insult you could ever use. Use this word with extreme caution for overexposure to its power could cause one to self deletus. Not safe for normies. The only comeback to this life crumbling roast is "no u"
Greg: *a well constructed insult*
Jarry: Ur mom gay
Greg: *immediately dies and WW3 starts while the Russian national anthem blasts with the bass turned up to maximum volume*