Like women, men have dozens of choices when it comes to their underwear. For a guy, one of the main features that can make or break an underwear purchase is the fly (the opening in the front of the underwear). Traditionally, the fly on men's underwear has been vertical. As of 2010, however, the horizontal fly is becoming popular because of its advantages.
Traditionally, the fly on men's underwear has held a simple double cross-over design, according to Hisroom.com. In other words, the fly basically has been two flaps of cloth that folded over each other. In theory, these flaps were supposed to cover the man's privates without a problem. In practice, however, men found that the the flaps "gapped," leaving them embarrassingly exposed. With a horizontal fly, there still are two flaps, but gravity does a much better job of holding the top flap in place over the under flap. The result is less gapping and increased security.
A traditional vertical fly runs in the same direction as the male genitalia. This means that the seams and ridge created by the two flaps is in contact with a large amount of the man's body. This is not good from the comfort point of view; more ridges and seams mean more friction that can chaff and irritate. In contrast, the ridges and seams for a horizontal fly run across the genitalia, leaving less area prone to irritation. Depending on how active a man is, this can make a huge difference in how the underwear feels.
Definition and Support
Hisroom.com points out that the standard vertical fly that features a double criss-cross design has a tendency to flatten out a man's natural shape and contours. The standard vertical fly also doesn't provide much support. A horizontal fly, by contrast, usually is paired with what is known as a "contour pouch." This pouch and the way it functions with the vertical fly supports a man better than a vertical fly does, giving him more definition. The subtle support that comes with the horizontal/comfort pouch combo also helps a man feel more "in place." This can make some men feel much more confident.
Because a horizontal fly does not follow the genitalia line and provides more natural support and silhouette, the man has more options about what he can wear. The increase in clothing choices, in conjunction with the natural definition provided by the underwear, ultimately can make the man look more attractive.