What would you do if some guy exposed himself to you on a trail when no one else was around?

I was with my husband and child and this guy totally whipped it out(so says my husband.) I did not look. This guy was GROSS and he got off his bike, stopped ,started coming his hair and started messing with his pants as we approached. Luckily my kid was asleep. This trail has tons of bikers and runners on it, but also alot of homeless people too. It makes me SO MAD! Cause I love to get excercise and I love nature. It is so not fair.