Why is the lemon curd metallic? How to fix it

I used this recipe and followed it to the letter. I used organic/unsprayed lemons (and was careful not to zest the pith) and free-range eggs. I did a quick Google search, but the only cause I could find was using a reactive saucepan, which couldn't be the case because both the whisk, the strainer, the bowls, and the pan were nonreactive, stainless steel. Yet the lemon curd ended up with a metallic aftertaste, that got stronger as it chilled.

Why was my lemon curd metallic, and is there any way to fix it? How?

3 large eggs, or 1 large egg plus 3 large egg yolks
Zest of 1 medium lemon
1/2 cup strained fresh lemon juice (from about 3 medium lemons)
1/2 cup sugar
6 tablespoons unsalted butter, cut into chunks

1. Set the strainer over a medium bowl.
2. Whisk the eggs (or egg and yolks) in a small nonreactive saucepan to blend. Whisk in the lemon zest, juice, and sugar. Add the butter. Whisk over medium heat, reaching into the corners and scraping the sides and bottom of the pan, until the butter is melted and the mixture is thickened and beginning to simmer around the edges, then continue to whisk for about 10 seconds longer.
3. Remove from the heat and scrape into the strainer, pressing gently on the solids. Scrape any lemon curd clinging to the underside of the strainer into the bowl.
4. Refrigerate until chilled before using. Lemon curd keeps in a covered container in the refrigerator for up to 1 week.

EDIT: I made a new batch, using 1 part lemon juice to 1 part sugar plus the zest from the lemons, and the same amount of butter. I omitted the egg yolks and used cornstarch instead. The lemon curd came out perfect, so it must have been the yolks. Why?