הלכה על יבמות 119:12

It is from the roots of the commandment [that it is] since the essence of good in a person is that he have thoughts of purity and cleanliness - for after the thoughts are the deeds of bodies drawn. Therefore it is fitting for the highest servant to cling to a woman who has never fixed her thought towards another man besides him who is Holy of Holies. And through that, the seed that God, may He be blessed, gives him from her will be pure and clean; fitting to serve in holiness. And lest you say, "And who knows if the virgin has also fixed her thought towards a man besides him and set her eyes on another," the answer to this is that for as long as the thought has not gone from the potential to the actual, she is not disqualified. But anytime she had intercourse with him, she is disqualified. And even though they, may their memory be blessed, said (Yevamot 59a) that when she becomes an adult, she is prohibited to him, the matter is that once she has matured so much, the thought of her impulse is only evil; and perhaps she fixed her thought towards another. And from when she is an adult, her evil thought counts as an act. And so [too,] from this reason did they say (Yevamot 59a) that if she was widowed from betrothal - even as a minor - she is forbidden to the [high] priest. As from when the act of designation (kiddushin) was done with her, she already fixed her thought towards another man via the act of betrothal and was [so] disqualified - as an act disqualifies even with minors, and thought [does] with adults. And likewise they said (Yevamot 59a) that if she had intercourse not in her (customary) way, she is also disqualified - as she already did a big act, even though her virginity (hymen) is intact. And so [too,] they said (Yevamot 59a) that even [if she is one] struck by wood (on her hymen), she is disqualified. And the reason with her is apparently that she will not fix her thought greatly on the high priest. As once she lost her virginity, she does not cut (form) a strong covenant with a man ever. And [this is] similar to that which they, may their memory be blessed, said (Sanhedrin 22b) that a woman only cuts a covenant with one who 'makes her a vessel' - and behold, this one did not 'make her a vessel.' However they, may their memory be blessed, were not very stringent with these [laws], and they said (Yevamot 60a) that if he married an adult or one struck by wood, he keeps [the marriage], ex post facto.