Does sexual fantasy that results in clear, viscous discharge constitute masturbation and necessitate ghusl?

If fantasies overwhelm a person involuntarily, without being sought out, then he is not held accountable for them. However, if one deliberately seeks them out and indulges in them while being able to resist, then it is feared that this may be a sin. Therefore, one should abandon these fantasies and combat them, as they may lead to bad habits. The secretions resulting from these fantasies are often madhy (pre-seminal fluid), which nullifies ablution (wudu') and necessitates purifying any part of the body or clothes it touches.