Why is a wife who engages in extramarital relations guilty of adultery, while a husband who does the same is not?
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For whatever reasons, Judaism originally allowed a man to have more than one wife, whereas a woman could only be married to one man. Therefore, as soon as a woman was married, having relations with any other man would automatically be "adultery." However a married man could in theory run off and marry some other woman in addition to his first wife; we'd call this "polygamy", not "adultery." So even if there was no marriage between them, we couldn't call it "adultery" because there was no rule of "no other women" on this man.
Clearly, if a married woman has consensual relations with a man who's not her husband, both of them are deemed equally guilty and have equal punishment.
Now today Judaism has banned polygamy, and a couple goes in with the understanding that it's just one man and one woman in this marriage. A husband who cheats on his wife with a single woman has done a horrible, disgusting thing and will answer to G-d for his actions, but we can't technically call it "adultery" the same way if the genders were reversed.