Should I tell my fiancée that I’m bisexual and had sex with her dad?

I have been in love for the last year and recently proposed.

After 10 years of bisexuality (though I had more female partners than male) I was happy, excited and in love.

She introduced me to her parents three months ago and her father and I recognised each other from a local cruising site.

We have been intimate on about three or four occasions.

He has made contact with me and asked me to tell my fiancée about my sexuality.

I asked if his wife knew about his, and we have reached something of an impasse.

He has since taken screenshots of old photos of me and I have also found some of him.

Shall I just walk away from the woman of my dreams?…