That’s a good attitude to start with, but not necessarily applicable in all circumstances. If, for example, (and this is just an example, obviously) I’ve tried sex with much younger women and found that, for whatever reason, I don’t enjoy it as much as with women closer my own age, I don’t think it’s out of order to say ‘no women under 40’. Similar, if a fella finds - after experiencing slimmer women - he enjoys sex more with larger women, I think it’s perfectly reasonable to say as much.
So, prejudging, yes, I wholly agree; it’s daft to shut off opportunities/new things, but it’s not daft to conclude after experiencing that you don’t like x or y.
(honourable exception for straight/gay; I wouldn’t ever suggest to someone who’s gay that they just try the opposite sex, nor someone who’s straight to just try someone of the same sex.)