It's not often you see men wearing hot pink. There's nothing with those that do - it can look great as part of the right outfit - but it's historically more the sort of color a woman would wear. For that reason, men often laugh at other men they see wearing pink. If anyone was laughing at this older gentleman, the laughter would soon dry up when they see what's written across his shirt.
The implication here is that the t-shirt doesn't belong to him; it belongs to the girlfriend of the reader. This is the kind of shirt you'd expect to see a college jock wearing, as a brag about his success to women. It's the fact that we're looking at a retiree wearing it that makes it shocking - and all the more hilarious as a result.