All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Thinking about marriage doesn`t invalidate ablution save when there is lust that causes a discharge of sperms. In this case, ablution is invalidated since whatever is discharged from the two private parts invalidates ablution. However, a person should avoid thinking about the aforementioned matter since Allah, The Most Exalted, said (What means): "For every term there is a Book." [Ar-Ra'd/38]. This verse indicates that everything has a specific period of time that none knows but Allah. In addition, the Prophet (PBUH) has advised the one who's incapable of getting married to fast since it's the only method that helps overcome such thinking. Yet, too much thinking of marriage would cause obsessive compulsive disorder. Therefore, it should be treated from the very beginning to avoid such an illness. And Allah knows best.