Thrush can sometimes be transmitted during sex however thrush is not a sexually transmitted disease per say.
You can get thrush even when not having sex or being sexually active.
Sex and Sexual activity can make thrush symptoms worse though so if you have thrush you should avoid sex or sexual activity until the thrush clears up.
Thrush is a fungal infection that is caused by causative fungus, Candida albicans which is a natural inhabitant of the mouth and throat.
When your bodies immune system is depressed or not working properly then the Thrush can flare up and while Thrush is not considered to be contagious you should avoid having sex or sexual contact with someone while you have thrush to avoid worsening the thrush symptoms.