Believe it or not, Holden devotes significant text time to telling us about Stradlater. He's an athlete, he's handsome, girls really go for him, he's got a nice body, he's always walking around in a towel to show off that body, and he's a "moron."
On the other hand, sex is definitely Stradlater's forte. His technique is coercion. As far as sex goes, Stradlater is the opposite of Holden. We get to see this contrast play out over Jane; look at how each of them think and talk about her. Holden focuses on the emotional, on their friendship, on Jane's unique qualities as a person. Stradlater doesn't even know her name. All he's concerned about is the fact that she only signed out until 9:30pm. If we were Holden, we'd be worried about this date, too.
Even worse (in Holden's mind, of course) is that Stradlater is one of the phoniest phonies he's ever met. Sure, he's good-looking on the outside, but Holden deems him a "secret slob" because his razor blade is always full of hairs and junk. He's obsessed with appearances (he spends a ton of time standing in front of the mirror), but there's not too much under the surface. Sounds a lot like what Holden doesn't want to be when he grows up.
Of course, we have to ask the ever-popular question, can we trust Holden's descriptions? Is this guy really so bad? We might be able to get Stradlater off the hook (as we did for Sally) for simply being an average guy. But he's still not a guy you'd want in the backseat of a car with a girl you care about.