If you're just using someone for sex then why should they tell you? If you're physically attracted to them then what changes? If someone comes up to you that you find really attractive and gives you a bj and you enjoy it, what's wrong with that? You had a boner, found the person attractive, and enjoyed it.
Does your semen rewind across the room and back up your dickspout the second you hear they have a penis between them thighs that contributed to your stiffy?
If you're just in it for sex, and you're attracted to them, then what's the big deal? It's not like you're sharing your lives together. You're getting off, whoop-die-doo