Even Greener Pastures

I can't back it up with rules, but for me it would be natural for me to say he's vegan, he's a vegetarian, he's gay.

Ha! That's just what I was thinking. Vegan, a vegetarian, gay. Indefinite article in front of vegetarian, but not in front of the other two. I have no sensible explanation for this. I thought that maybe my non-native intuition was playing tricks on me.