When you have a "friend" sleeping in bed with you and you are "crossing borders" by having sex, feeling eachother up, etc. you on one side, your "friend" on the other. then you cross borders, that the girlfriend sets up, and "do it". usually tiedown free.
Danny and Jade wanted a little fun, but Jade didn't want him crossing borders.... He did anyway, now she's knocked up.
By Gray
Crossing The Danish Border
To switchchange sexual preferences from any category or sub-category to something new.
Dan just crossed the Danish border when he went home with that girl. He has been gay for years.