I, the female, have full custody of my child who is 15. The x sees him about once a year and I don't think he'd kick up to much trouble over it anyway, but could this cause problems for me in regards to keeping custody of my son? How would I explaine to the Judge that our girlfriend lives with us?
Yes, it could cause problems. Depends on the law where you are. You may want to search this forum as this particular challenge has been discussed at length and is a very serious concern.
As a non parental husband I don't have a problem telling people I am open. I just tell them and they accept it. I haven't hit any serious back lash personally.