Seinfeld is a "classic" 1990s situation comedy from the US that takes place in New York City.
There have been many American phrases coined from that show.
The "Today's Sponge" is a contraceptive piece of foam with a spermacide on it. If a woman felt she may have the need to inhibit fertilization of her ova from a gentleman caller, she could insert the product in the morning and have it all day so when the time came, she would not have to apply it later.
In the mid 1990s the sponge was taken off the market by the US government's Food and Drug Administration on concerns that some women were suffering from Toxic Shock Syndrome, a condition usually related to leaving a tampon in for an extended period of time.
In the TV show, Seinfeld, Ellen knew they were going off the market so she stocked up. Since they were in limited supply and you would wear this all day, she felt that a man had to be "sponge worthy" in order for her to use it. Because if she uses one, it diminishes her limited supply.
Two scenes about it.