eat asparagus. Than she will want the soapy taste back ;)
Your girlfriend is likely referring to the smell of the chlorine contained in the semen, chlorine is a natural component of semen and is often in a large enough concentration that some people can detect it's odor. It's completely normal and there is nothing you need to do about it.
Some reports say the consuming a diet that is high in fruits; especially watermelon and pineapple; and low in meat protein can improve the taste of semen. Avoiding alcohol and smoking cigarettes has also been reported to have a positive effect.