Does the fluid that exits before semen invalidate ritual purity?

Most likely, what exits before semen is madhy, which is a white, viscous fluid that emerges during foreplay or when thinking of intercourse. One must wash the penis and whatever of the clothing it has touched, and perform ablution (wudu), based on the Prophet's (peace and blessings be upon him) saying: "Perform ablution and sprinkle water on your private parts."