It's a wrong done already by 2 adult. Sure both are suffering.
Don't abort it. It's life.
Recommend both of you sit done & talk.
Get these facts clear without hurting her:
1. Is it's really yours?
2. How both decide which path to take down your life.
either: she & her husband brings the kid up? (Hard for both to
face for years to come) You can choose to leave & don't ever
come back, bring this secret to your grave. (This makes you a
heartless person) Never to speak to anyone cause it will cause her,
her husband & kid a lot of problems.
or : she divorce & follow you? (This makes you a Family
Breaker) Will you be happy with this women since she is unfaithful
type?
3. She divorce & you don't marry her. But both you & her
have the responsibilities towards this kids. This makes you a
Family Breaker also, & a single-parent creator.
4. It's time both of you should wake up your "fantasy",
responsibities & guilt is here to stay. You will need to face
it.
Rest assure your case is not uncommon. From ancient time till to
date, these problems have been occuring a lot. Just we done usually
hear about them.