You can thank sodium laureth sulfate, also known as sodium lauryl ether sulfate (SLES), or sodium lauryl sulfate (SLS) for ruining your drink, depending on which toothpaste you use. Both of these chemicals are surfactants — wetting agents that lower the surface tension of a liquid — that are added to toothpastes to create foam and make the paste easier to spread around your mouth. They're also important ingredients in detergents, fabric softeners, paints, laxatives, surfboard waxes and insecticides.
While surfactants make brushing our teeth a lot easier, they do more than make foam. Both SLES and SLS mess with our taste buds in two ways. One, they suppress the receptors on our taste buds that perceive sweetness, inhibiting our ability to pick up the sweet notes of food and drink. And, as if that wasn't enough, they break up the phospholipids on our tongue. These fatty molecules inhibit our receptors for bitterness and keep bitter tastes from overwhelming us, but when they're broken down by the surfactants in toothpaste, bitter tastes get enhanced.
So, anything you eat or drink after you brush is going to have less sweetness and more bitterness than it normally would. Is there any end to this torture? Yes. You don't need foam for good toothpaste, and there are plenty out there that are SLES/SLS-free. You won't get that rabid dog look that makes oral hygiene so much fun, but your breakfast won't be ruined.