Lots of people assume that monogamy equals a lifelong committment. Sure, if that assumption is examined, the two are not automatically connected. But it's a common idea in the mainstream. I've also heard many people, some divorced, some widowed, say that they are just not interested in committing to another lifelong, or even long term, relationship.
I think it's a good thing you realize this about yourself. Yes, you may find that being up front about it limits who might respond to you. But ethically, it seems like the right thing to do.