[Torchwatch]

If you are running and swimming regularly than you are likely to be fitter and slimmer than your "twin" who watches TV and drinks beer for 8 hours a day.
Darwinian evolution considers survival of the fittest, he who is fittest will tend to attract the best mates and have the opportunity to create offspring as widely as possible.
Imagine that you are your "twin" are invited to a pool party on a warm summers evening. You turn up in fairly short shorts but with pockets and a clean white tee shirt while he turns up in a black Status Quo tee shirt and jeans with a heavy belt.
Your hosts are busy laying out the food and ask if you would like a swim or if you'd prefer to have a drink. You quietly strip off your shorts and tee shirt revealing a pair of red 2" side speedos and slip into the pool, joining the younger and fitter guests already there. Meanwhile your twin graciously accepts a beer collapses into a garden chair and commences on the nibbles.
When the food is ready and everyone is queuing with their paper plates you dry off and drape a towel over your shoulders and join the queue. By now you are hungry and have an appetite and fill your plate choosing chicken pieces, a little quiche, some salad and a wholemeal roll washed down with a glass of Australian Shiraz. Your "twin" goes for a couple of big burgers, a hot dog and a handful of those tasty cocktail sausages. A couple more cans of beer help him eat fast enough to keep up.
Once your hosts have finished eating and clearing away they change for swimming, he wearing a reasonably short pair of swimming shorts, she in a speedo woman's one piece. Already in your speedos you just slip back into the pool, meanwhile your twin heads into the house to get changed, he returns in a pair of brightly coloured board shorts complete with a floral pattern. These shorts go down below his knees emphasising his plump calves while his belly hanging over his waistband, you hear someone mutter "cupcake" and can't help your self smiling.
Tonight both you and you twin find a casual partner of your preferred sex, for you it was your trim body in those red speedos that helped, for your twin it was the fat bird that was looking lonely. In the quiet spot among the bushes at the bottom of the garden your partner can see and feel that you are horny and the two of you let nature take it's course, in the spare bedroom your twin and the fat bird are snogging, suddenly your twin farts loudly and doubles up with indigestion, the beer he has drunk has made it's effect on him and he simply can't get it up.
Of course guys that wear speedos and short shorts are more horny, on average they are fitter, slimmer and healthier and this will make them hornier.