It's only unethical if you're using the person you're dating.
So let's say you make a pact with a gal pal to help you make Ms. Wonderful-but-not-as-wonderful-as-Mrs. Peel jealous. Not unethical, because she knows what the date is about, and has agreed to help you. All's fair in love and war, they say.
Now let's say you'd like to make Ms. Wonderful-but-not-as-wonderful-as-Mrs. Peel jealous, but you don't tell the girl you're dating. Unethical? It is if you have no interest in the girl you're dating. It's totally unfair to her.
But what if you want to make Ms. Wonderful-but-not-as-wonderful-as-Mrs. Peel jealous, and you also happen to like the girl you're dating, but you don't tell her that you're trying to make Ms. Wonderful-but-not-as-wonderful-as-Mrs. Peel jealous? Hmmm, looks like a GREY AREA. On the one hand, you owe it to your date to not have conflicting agendas. On the other hand, it's just a date, not a committed relationship, and you haven't led her on about anything. The key to keeping it ethical is to be fair to your date-- to not use her as a prop in your Piggly-Wiggly psychodrama. So you date her, and if it makes Ms. Wonderful-but-not-as-wonderful-as-Mrs. Peel jealous, great, but ONLY if you're genuinely interested in the girl you're dating. And that's the conflicting agendas part-- if it's not already unethical, it has the potential to become unethical. So unless you can be fair to your date, you shouldn't date her.
Now let's say that you meet a new girl you'd like to get to know better, because she may be somebody who could be a new Ms. Wonderful-but-still-not-as-wonderful-as-Mrs. Peel, so you ask her out, and you have no conflicting agendas. You genuinely want to go out with the new Ms. Wonderful-but-still-not-as-wonderful-as-Mrs. Peel. Ethical, even if, as an added bonus, the old Ms. Wonderful-but-not-as-wonderful-as-Mrs. Peel discovers she's jealous. Completely ethical.