Ok I see.
I meant on psychologically. The sexual urge can follow the presentation of an opportunity. Or not. However, according to serial killers themselves they are always thinking about it. However, I do think he was actively hunting that day and didn't mean to imply otherwise. As you said, his clothing is a strong indication of that.
Sex can be had anywhere for significantly less risk. It's the irresistable psychological urge that caused him to kidnap two teens in a public place in broad daylight with other people around. Whatever that urge is.
Studies have shown serial killers are always prowling either actively or passively. Kind of like a freshly fed cat's pupils will dilate if you pull a string in front of them. They may or may not chase the string, but the urge is always right under the surface because they're predators.
Edited to say I didn't mean to imply a killer strikes each time an opportunity presents itself. They'd all be caught if that was the case. Don't we wish.