I think saying “I’ve had impure thoughts” would suffice. There is no need to go into graphic detail as an impure thought is an impure thought regardless of the actual content.
Such thoughts are only sinful if one deliberately entertains them. Thoughts will enter one’s mind uninvited and that is to an extent entirely natural and in no way sinful. At the very most it would be a venial sin. However venial sin can quickly lead to mortal sin if not tackled.
It is therefore good to confess such thoughts in any case as by going to confession and confessing them, Our Lord will grant you the graces and strength necessary to help prevent those thoughts from leading you into sin.