Order.
Okay so here's the thing. Most of us can't personally know everyone. So when someone else in this group is going 'hey, I have this friend who is really good at X and can help us with this task' we're trusting your judgement. We're trusting you to disclose any relevant information that we might need to know or plan for or might sway our decision on whether or not to trust that person's involvement. Even if it's a limited task.
Willingly sleeping with a convicted Death Eater and intending to marry said Death Eater absolutely falls into that category. That doesn't fall into 'a secret to respect her wishes about' when we're deciding whether or not she should assist with an Order plan. That's something that we need to know about, period. Her increased exposure to Death Eaters, even without considering the rest of this, is something we should've known about.
I don't know why it is that you feel you can't tell us information, and I really hope it's a reason better than because you're afraid someone's going to disagree or be mad at you. Because keeping this sort of information from us is incredibly dangerous and leaves us vulnerable when it doesn't need to, and it's not worth someone dying over.