In Srila Prabhupada’s purport to the Srimad-bhagavatam verse 3.14.21, he writes, “In other words, the so-called man of the renounced order, who has left his house and wife, goes to hell if he again desires sexual pleasure, knowingly or unknowingly.” What is meant by “knowingly or unknowingly”? How can one desire unknowingly?
One may lack deep self-awareness and consider one’s impure enthusiasm to engage a woman in service to be pure bhakti.
What is implied or gained by engaging a woman in service that makes one’s enthusiasm impure? Her association? Her affection?
It’s just as you surmised. One may become attached to the opposite sex in the name of Krishna consciousness.
Would it be better to not engage women at all if a sannyasi is still impure or better to engage women and be honest that one is dovetailing and still purifying the heart?
No, it is better to engage everyone but to be conscious of one’s motives so that one can purify them and advance spiritually. Of course a sannyasi should avoid situations where he simply can’t control himself.