Generally adolescents starts meeting and going out with the same gender initially.
During higher secondary and college age, boys as well girls may start going out for picnic or movie or for celebrations together.
At a later age, when the friendship becomes more close and one starts liking one another, the two become company for them.
Going out together and passing time together is dating. Dating does not always involve sexual relationship.
Parents may know the friends of their adult son/daughter better. If possible invite them at home for better understanding.
Young adult are likely to go out with their friends including opposite gender, which is a natural phenomena.
Instead of having rigid rules, try to discuss out the matter.
Please remember the son or daughter is going to spend the lifetime with opposite sex after marriage, so try to answer their curiosity now.