The other responses in this thread are correct, but I suspect may have still left you confused.
You are correct that 50% is always 50%, but employee stock options are typically given as a number of shares, not as a percentage of the total, which is what allows for dilution. At least, that's how I understand it.
Any number of shares at any given point is a percentage of the company, and any percentage is always a certain number of shares.
You won't stay at that percentage though, unless the founders agree to protect you from dilution. In that case, they'd compensate you with some of their own shares whenever you dilute to keep your percentage the same. It's not common for employees though.
The point is that dilution works by creating new shares out of thin air -- the total number of shares increases. Meanwhile, the total percent remains 100%, for obvious reasons.
Sure, but that doesn't change whether you're giving out a number of shares or a percentage, because both are the same, just expressed differently. There's no common theme where somebody says "here are 0.1% of the company" and implies anti-dilution. It's just easier to talk about percentages than shares, because it's easier to calculate if you know the total valuation. You'll dilute in both cases.
You are correct that 50% is always 50%, but employee stock options are typically given as a number of shares, not as a percentage of the total, which is what allows for dilution. At least, that's how I understand it.