Its perfectly fair to him. If you believe you are old enough to have the sex, you are old enough to deal with the consequences of the sex. Sex is, after all, for REPRODUCING.
And if you're into the whole pro-choice thing: It's your body...not his...
However...if you did do it on purpose behind his back....that's seriously fucked up, and if he leaves you....i would say you deserve it.
What if he did something behind YOUR back? I cant think of an example since men cant get pregnant....
Seriously tho....tha f**k?
Having a baby never "saves" a marriage. It just delays the inevitable. Especially when one party did not want the child.
That's not to say I have the answer about what you should do about it. It's on you and what you think is best.