While selecting targets if Houthis attack the airbases that these planes are attacking from then it is a defensive response which cannot trigger a defense agreement. That is prioritising targets where attacks are originating from.
When they are attacked by saudi they need to prioritise the military ones and airbases where the saudi attacks are originating from. Ammo depots , supply channels-bridges or saudi proxy groups bases inside saudi arabia are other valid targets that directly supports saudi attack in current Yemen conflict. Oil also supports the war and for example Ukraine strikes Russian rafineries for the same purpose but this should be a later target if the war escalates further which can trigger defense agreements if used now.
Economic targets like aramco should be spared as deterrence for later options like if other countries intervene after Houthi defensive-strikes against saudi military targets.
