All perfect praise be to Allah, The Lord of the Worlds. I testify that none is worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Provoking sexual desire only does not invalidate fasting or entail offering expiation with greater reason. However, fasting becomes invalid if one intentionally causes himself to experience a sexual discharge. As for breaking fast by intentional emission of mathy (i.e. pre-ejaculatory fluid), scholars hold different opinions regarding it.
We would like to draw the attention of our questioner to two important points:
Firstly, if a woman becomes pure from menses during daytime in Ramadan, she is not obliged to fast the remaining part of this day.
Secondly, she should not use the blessing of thinking, which Allah, The Almighty, has granted her, except in matters that involve benefit to her concerning her religion or the worldly life.
Allah Knows best.