That's not correct, and your reasoning is entirely flawed. Your premise that a product is even if and only if both $x$ and $y$ are both even or both odd is wrong. Two odd numbers, when multiplied together, always result in an odd number. A product $xy$ is even if either $x$ or $y$ is even, and that's how you can arrive at the $\frac{3}{4}$ probability. But I need a proof for this, so please formalize this fact in a proof.