I saw my son and daughter french kissing?

I got home from work and they were in the living room behind the sofa they were both lay on their side and they were kissing i stopped them and sent them to there rooms but after i told my wife we did not know what to do my wife has since seen them doing sexual things with each other again but do i discipline them or not i have told them its wrong but they don't seem to care and they can see what i am saying because my son is 15 and my daughter 13 i don't know what to do