is this unethical?

I am currently applying to a summer program, and in my application I’m talking about how much I “truly” want to attend it, “extremely interested,” please “seriously consider me” etc. However, in reality I also applied to a second summer program, and if i get accepted there I most likely would actually attend that one. Is it unethical for me to tell this program that it is my first choice, when really it’s my second?

You can be seriously interested in two programs. If you like the program, there’s no crime in telling them that you like them. That’s pretty much how you write summer program applications.

Anyhow, people reject admissions from programs for a variety of reasons.

It’s fine.