Also, sometimes (not always) it helps to have an actual person in the picture.
Like, if you meet someone, and you bring her home and let your mom get to know her as friends... then when you mention that this person is more than friends, it's possible she'll think of her as the person and not just some nameless, faceless marriage wrecker.
Besides, it's often best when relationships start more friendly than lusty. So the whole "bringing her over as a friend" doesn't have to be a sham or degrading to your girlfriend.
I would never want to date someone who is completely in the closet. I don't like feeling like a dirty little secret. But I totally understand how it's different with family, and how that can really mess up the rest of your life if they give you a hard time. So while I want to be able to hold hands when we go out for a date, I don't get offended when I'm introduced as a "friend" to my partners' families. But if they introduce me as a "friend" to their best friend, I might be a little more wary.
“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