“pretty wrongs that liberty commits”
'Pretty' here means small, trivial, or slight — not physically attractive. 'Liberty' means licence or unchecked freedom of behaviour. The speaker is calling the Youth's infidelities charming little trespasses, diminishing them even as he names them.
“Gentle thou art, and therefore to be won, / Beauteous thou art, therefore to be assail'd”
The two lines repeat the same logical skeleton ('thou art... therefore to be...') with different adjectives — a figure called anaphora. 'Gentle' here means of gentle or noble birth, refined and courteous; in Elizabethan usage such a person was thought naturally yielding and tractable, hence 'to be won.' 'Assail'd' carries a siege or military sense: beauty invites attack as a fortified town invites assault.
historical The sonnet form“when a woman woos, what woman's son / Will sourly leave her till he have prevail'd”
'Woman's son' means simply any man, since all men are born of women — the phrase argues that no male could resist a woman who actively pursues him. 'Sourly' means grudgingly or reluctantly. The sardonic logic completes the excuse: the Youth was importuned, and resisting would have been unnatural.
“forced to break a twofold truth:— / Hers by thy beauty tempting her to thee, / Thine by thy beauty being false to me”
'Truth' here means pledged fidelity or troth — a promise of faithfulness. A 'twofold truth' is a double pledge: one owed to the woman (whose desire for the Youth the Youth himself inflamed) and one owed to the speaker (whose partner the Youth has taken). The closing couplet turns the Youth's beauty into the agent of both betrayals — the same feature that attracted the woman and made the speaker love him is what destroyed both relationships.
historical The Fair Youth, the Dark Lady, and the Rival Poet