By getting the girl to have a shower, it makes them already naked, and you don't need to do the task of seducing* the clothes off the girl, which normally consists of one piece of clothing at a time, could take a lot of time and effort, whereas the shower skips all the hard work and you get your sexy time quicker. Commonly abbreveated to ER. Once again all this could also be avoided by slipping viagra into her drink
*seducing can include anything from kisses to massaging to ripping. Depends on the girl, and your relationship with her...
ps. This is only the case if your girlfriend is not super horny and is the one taking your clothes off... in that case *hi5* man!
"Bernie told his girlfriend to have a shower so he could take the express route"
"eh i can't be bothered kissing you, go have a shower.."
"Mat couldn't get this girl to take the ER, so he had to seduce (rip) her clothes off" naughty boy mat...