Adapted from a recent online discussion.
I am having sex with three people right now. (Not literally, but you get the idea.) Two of them have no idea there is anyone else and the third one might. My friends say I'm being deceptive, but I say it's fair game as long as I don't pretend I'm being exclusive and we always use protection. Who's right?
My evil twin hopes you fall hard for someone who treats you the same way you're treating these people.
The answer to your specific question is that you're telling a lie of omission. If you fear the truth would upset them, then you're being deliberately deceptive.
Certainly if your sex partners are telling themselves they're in an exclusive relationship, then they need to take better care of themselves. However, I equate their mistake to letting an insurance policy lapse. That makes you the natural disaster for which they failed to prepare.
So, repeating my answer, because it's my party and I can harp if I want to: No, it's not okay to go out of your way to treat other people badly-- especially not with something so trust-dependent as sex, but not even with something petty, like zooming up the breakdown lane to get ahead in a traffic jam. Stop seeing your needs as special and others' needs as mere obstacles to getting what you want. It doesn't get you ahead, it makes you a jerk.
Seriously? Every sperm is sacred? Sure s/he should say they're not exclusive, though it doesn't sound like that is assumed anyway; besides assumptions are for [fools]. Why all the hate? Carolyn, I'm disappointed in you.