Well obviously it's possible to hold both to account, because so many people do. But believing that Gemma is at least partly accountable for Jake's marriage to Gina doesn't make it true. Gemma might be an absolutely irredeemable cow who's never happy unless she's shagging someone else's husband, but ultimately these vows aren't hers, this commitment isn't hers, and she isn't responsible for them. Jake is, and nobody else.
It doesn't mean that Gemma isn't a shit, but it does mean that only Jake made a commitment.
There are all sorts of reasons why I think it's a terrible mistake to hold Gemma responsible for Jake's commitments (even if you are ALSO holding Jake responsible), and I won't go into them in detail here. I'll just say that when decisions are being made about a marriage, you simply cannot be factoring other people into it. Either you trust each other and your commitment to each other, or you don't.