why does hamlet blame beauty for the loss of a women’s honesty?

This is without a doubt one of the more difficult concepts Hamlet delivers in the whole play. In this passage honesty=chastity and fair=beauty:
HAMLET
That if you be honest and fair, your honesty should
admit no discourse to your beauty.

OPHELIA
Could beauty, my lord, have better commerce than
with honesty?

HAMLET
Ay, truly; for the power of beauty will sooner
transform honesty from what it is to a bawd than the
force of honesty can translate beauty into his
likeness: this was sometime a paradox, but now the
time gives it proof. I did love you once.

Hamlet is telling Ophelia that if you are both chaste and beautiful, your chastity should not permit anyone to converse with your beauty. Chastity keeps away the lure of beauty that would draw it into bawdy acts, i.e., sex. Ophelia's reaction proves Hamlet's point, beauty has no interest in guarding chastity though by its very nature chastity has to guard beauty. Beauty has such power that it would sooner turn chastity into bawdiness, rather than chastity forcing beauty to be chaste. In other words for Hamlet, beauty is stronger than chastity though he used to believe otherwise "sometime a paradox", his mother's behavior proves it "the time gives it proof".

As Hamlet is talking to Ophelia his mother's bretrayal to him and his father presses on his brain. What he sees in Ophelia is a future Gertrude with him as her victim. But, let's back up a second and focus on an interesting word Ophelia uses; "commerce". Here we are drawn into a subtext that is often mistaken for the main point of ths scene. Go back to 1.3 where Ophelia speaks of how honorable (equate that with noble) Hamlet was in words of affection to Ophelia as she tells her father of their budding relationship.

The subtlty of the word "commerce" connects with "bawd" which then gives greater depth to the use of the word "nunnery". But again this is an undercurrent of the scene that is more an indictment of the Queen than Ophelia. The direct use of the term "nunnery" ties back to Ophelia's pious action in devotional prayer. The use of "nunnery" as a brothel should not bury its literal sense as a place for nuns. A place that encourages prayer and where chastity is not endangered by beauty. And as Hamlet continues in his rant it protects her from the evils of marriage and the weaknesses women are inclined to, such as face painting, jigging and ambling, etc.

Hamlet is not pleading to save Ophelia, rather his tone is for her to wise up. Men are "arrant knaves" who don't love. We lust. His words of so sweet breath composed were not based in love but lust. She should not have believed him. Virtue cannot "innoculate" against our nature. Men are sinners. Why would she want to breed the likes of him. She should just get herself to a nunnery before she destroys herself and him along with her. This scene parallels and foreshadows his confrontation with his mother in 3.4 where the tone is the same.