I've been lying to my fiance about my son for almost 8 years.
Wait, what about your son haven't you told her?
He's not my son. He's actually my secret lover.
That must be a really hard secret to deal with but you've been keeping it a secret from your wife for almost 8 years. Had you told her about it right away you might have been able to salvage something, but because you kept this a secret for so long if she finds out now I'm sure your relationship is over. Don't keep major things like that from your partner or they just get worse and worse over time.
I don't care. My moment of bliss is more important than anything else.
You shouldn't lie to your partner. It will create big problems in your relationship.