WHAT?! I hear you say. These are the same people who put on shows with models we can't ignore! But that's the point, men. You can't ignore the models: but they want to sell the clothes. And will you buy them? You'll want to. You will see some really sexy number that would look just great on your current girlfriend: but then you face the basic problem.
The fashion industry has no sizing.
Sure it does, people respond. But put two or three women together and ask them about sizes. They will say that it depends on the manufacturer which size is which. And in your girlfriend's closet, should you look, you will find a range of possible sizing numbers, all of which she wears to great effect (except for that ugly poncho thing -- the only thing without a size). Suppose you decide to standardize: you're going to buy her a swimsuit you'd like to see her in. You pick the middle size indicator as safe.
The sizing for swimsuits is not done using the same scale as the sizing for dresses. Or pants or shirts, for that matter. And none of the sizes you are used to apply.
OK, you've learned. Don't buy clothes for your sweetheart. If you have children, you will be very helpful for the boys: but don't even think of attempting to buy something for your daughter. The same problem applies in spades. "She's a 6X" your wife will tell you -- but you've neglected to mention you're after a swimsuit, and no clerk in the store has a clue what 6X means in swimsuit sizes. Oh, sure, they'll ask stupid questions in an effort to be helpful ("Is she a big girl?" -- what does that mean in a 3 year old? tall? fat? chubby? for her age, or her size?), but they have no clue either.
Result? Gentlemen, buy flowers, candy, movies and a meal. Let her handle the rest of the stuff. Sure, you'd have some suggestions, and some different approaches. Stick to things with sizes that have some primitive form of calibration (though how many bras, panties and shoes do you want to give her, really? Could this be the actual source of the stereotypical exploitation costume?).
UPDATE: Welcome, Carnival of the Capitalists readers!