In my case, it's a totally unexpected and powerful attraction to my mother-in-law. This has nothing to do with my wife, whom I love dearly; and in any case, I hardly think my mother-in-law has any similar feelings.
It happened the last time my wife and I were visiting my mother-in-law, who lives several states away. She's 70, but looks at least 20 years younger, with a very nice figure for her age and a lovely face. I've always liked her and gotten along with her very well.
The second day we were there, we were all invited to a party given by one of her friends. We went to our separate rooms to dress. I had to use the bathroom, so I went into the hall to reach it. As I did so, I passed my mother-in-law's bedroom. She hadn't closed the door all the way, and I saw that she was wearing nothing more than her white bra and panties. And what an effect it had on me!
Of course I quickly returned to my room before she saw me, but I couldn't get her out of my mind. She just looked so good, so sexy, so desirable! She was all I could think about at the party we went to, and I had to make a real effort not stare at her like some lust-stricken schoolboy.
When we got home, I was aching for an opportunity to go to the bathroom and masturbate while thinking about her. When I finally managed to do so, I got an extra treat. Both she and my wife were downstairs, so I had free access to her bedroom. We had all changed into comfortable clothes after coming home, and the panties she had worn were in her laundry hamper.
Felling both embarrassed and incredibly excited, I took her panties into the bathroom with me and held them to my face, smelling the fragrance of her pussy while slowly stroking my cock. I kept seeing her in her bra and panties, with full breasts and a lovely broad ass. Then I tasted the crotch of her panties, imagining that I was licking her pussy. The thought of actually having her was an incredible stimulus, and I came as I hadn't in a long time.
Since then she's been a regular fantasy of mine, and I look forward to our next visit, in hopes of repeating that experience. I wonder if this is unique to me, or do other sons-in-law have similar fantasies?