Who doesn’t love a good “your mom” joke?!
Tank tops designed for women to wear while exercising are kind of an entirely separate genre of humor. It’s a very niche attitude they’re tapping but it’s valid.
You’ve gotta put yourself in their position, though. Imagine you’re just trying to work out, concentrate on having good form, count reps, knowing it’s only a matter of time before some guy comes over to ask if you want him to show you how to do what you’re doing “the right way” or ask if you need a spotter or just be generally creepy in some typical fashion.
So one way to let people know what time it is and what time it isn’t, is to wear a shirt that pretty much says to go screw yourself.