Is it permissible to think about Al-Hur Al-Ain when one's wife abstains from intercourse, while still adhering to acts of worship?

It is not permissible for a wife to refuse her husband's right to conjugal relations without a legitimate Sharia-compliant excuse. There is no harm in a Muslim thinking about the houris (companions of Paradise), for they are among the blessings of the people of Paradise. However, one should not waste time on fantasies and thoughts; rather, time should be occupied with what is beneficial.