Well the first thing I need to say is that your boyfriend is cheating on his girlfriend if he's not telling her about you. Second is that if your husband starts seeing someone and doesn't tell her about you, then he's also cheating on her.
As for how you tell them, it's something you have to feel out with each person. You don't have to say anything on the first date. In both the mono or poly worlds, it's generally assumed when you start dating someone that they might be dating other people too. Every relationship gets to a point where you start thinking of someone as your boyfriend/girlfriend as opposed to "someone I'm dating" and that's the latest you want to tell them about your lifestyle. Preferably much sooner, and definitely before sex.
When I started dating my now-husband, I was dating a couple other people. I made it clear to my hubby that I was poly and that I intended to continue dating these people when we were in a relationship. I told him I would like to consider him my primary and he was honoured and accepted.
Now, I pretty much tell people as soon as I can slip it into conversation. I'm not sure how well that's working yet. I think that when I say "my husband" and don't mention "open marriage" they just assume I'm unavailable, and that's not helpful. On Facebook, I'm a member of some poly groups, and all my dating profiles say I'm in an open marriage. So as far as people on the internet go, they typically know about my lifestyle long before I ever meet them. I don't go to bars and stuff, so I don't have any experience with that kind of dating.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker