I guess it would depend on how "bad" it was.
Is it a matter of them being selfish? or lacking skills/experience but willing to find out what I like and put in the effort? or Is it we've tried everything and it is just no chemistry in (that department)?
The sound of crickets or watching paint dry???
If one finds them self fantasizing about past lovers or feels tempted to cheat in order to be satisfied then breaking up is much more dignified.
The reality is many people in such a scenario would choose to cheat rather than breakup or run down to the courthouse to file for divorce.
Just because sexual incompatibility isn't a "deal breaker" for them does not mean they're prepared to go without sex or live out the rest of their life sexually frustrated.
Cheaters in this situation are looking to complement what they have. They want to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side.
Most cheaters don't believe they will get caught so they "go for it".
It really comes down to each individual determining just how important "good sex" is to them for the remainder of their life.
Some people with regard to sex have a "take it or leave it" outlook.
Ideally one wants to be with someone where the desire and pleasure is (mutual). No one would intentionally pursue sexual incompatibility and frustration. Many believe you can "teach the mechanics" but ultimately "chemistry" can't be taught. It's either there or it's not.
Life is a (personal) journey.
Each of us gets to have our own "deal breakers'.