For me, those red flags are:
1. Someone who thrives on drama. If you ain't an actress, and actually acting, don't bother, because I won't bite.
2. I will try to explain this one as succinctly as possible-- someone who gives me a glare when I find the two of them off in a corner, and that glare says something to the effect of "Haha! He's with me now and I'm not letting you have him back!"
3. Someone who can't/won't find a babysitter so you can have an hour or so of adult time, instead of dragging all parties, including two kids under the age of 5, to a McYuck's playland to have a poly/BDSM conversation.
4. DADT--YMMV on this one but I don't do them because they feel dishonest to me and it also feels like the other partner(s) are getting the short shrift.
5. Someone who is wanting my guy as a fuck buddy and won't make the child-free time for me to ask what questions I need to ask in order for me to feel secure. Orrrrrrrrrr, boyfriend's FWB and I do manage an adult night out and she leaves me alone for 1.5 hours in a city an hour from home, dependent on her for transportation home, in a bar full of people I don't know, so she can go have a lap dance.
Sorry, this was, unfortunately, a partial rant. *sigh* I thought I had dealt with this baggage. Guess not.
I will also go along with meeting the partners. I INSIST on it. If it doesn't happen within a reasonable amount of time, we don't happen.