Shouldn't the husband/wife who cheated be the "homewrecker," not the boyfriend/girlfriend that they cheated with?

Edit: I've seen this pop up in here a bit so I wanted to mention it. The point of this thought isn't whether the person on the side (ie. "Assistant to the Cheater" or "ATTC" for short) is at fault or is/isn't an asshole for sleeping with the spouse (The Cheater). It's about who broke a vow. The ATTC may have know of The Cheaters preexisting relationship, or they may not have. Are they a douche for sleeping with a married person? Probably. Is some of the fault on them? Arguably, yes. But the ATTC isn't the one who broke a vow. They aren't the one who wrecked a home and shattered the trust of the marriage.