One of my friend went through the same situations. Her male friend invites her to hang out with him and his girlfriend. While the three are together enjoying a night out on town ,he completely ignores her and does all those intimate things with his girlfriend. She actually felt like a third wheel a lot of times.She said it was awkward to watch them for hours doing stuff in front of her.She wished not tobe invited. Now she politely denies the invitation if he is with his girlfriend.