I like all of those! I'll leave it up to everyone's imagination as to which one gets said!
Well, taking your pick from the lines above, my girlfriend is startled to suddenly hear a complete stranger say this to her. She glances at me, not sure what to make of it and then looks up at you.
You apologise for startling her and then say that you wouldn't have said anything but that you don't think that her little brother was going to point out that her swimsuit was riding up like that.
Little brother? You mean me!
My girlfriend adjusts her swimsuit and giggles a bit out of embarrassment but also amusement at your comment about me being her little brother. I try to speak up but my voice cracks as I speak:
"H--hey I'm her boyfriend not her little brother!"
My voice cracking doesn't really help matters and you both laugh, while I turn red with embarrassment.
You ask my girlfriend what her name is and she tells you her name is Emma.
"So Emma," you carry on chatting to her, "I'd better see you back to the changing rooms - there are a lot of weirdos and perverts about"
And the two of you start walking off together with me following behind! I try and listen in while fuming that you're chatting my girlfriend up. I'm too much of a wimp to say or do anything about it though. I can't hear the conversation but I can hear my girlfriend giggling repeatedly at what you're saying.
You come to a set of double doors and you hold the door open for Emma so she can go through. As she does - I watch as you make no attempt at hiding your eying up my girlfriend. Your eye wanders all the way up from her bare feet, all the way up her shapely legs and you take another look at that butt...