[shimmeringfaerie]

I think you definitely need to tell all your sexual partners if you are seeing other people sexually (you may not consider oral to be intercourse, but it still carries STI risks). Any sexual activity with another person carries health risks, even if you use a condom, and I think it would be wrong to not let both partners know the risk. And if one of your partners got a STI because you failed to inform them that you had other partners, then you could be held legally responsible and be charged.

What you don't need to do is let either partner know who the other person is or any specifics about your relationship with the other person if you don't want to; just mentioning that you are sexually involved with someone else is fine.

If one of your partners doesn't want to see you anymore if they know you are seeing other people, then that's their choice. There's nothing that you can do about it, except decide to be exclusive with that person if that's what you want.