You're not alone.  A similar thing happened to me, and for similar reasons.  When she finally decided to admit it, my wife told me she had 'been with quite a number of guys' over the years of our marriage.  When I asked her how many or who they were, she refused to be specific, but when I asked her why she said it was because she 'wanted more sex' and/or because she 'had lots of opportunities'.  She told she had a psychological thing about big cocks, and found it more difficult to resist if a friend of hers or the guy himself told her he was 'well hung'.  She then described in detail how she liked to be taken and how she felt when these other guys were fucking her.  Perhaps I should have been mad, but instead I got extremely aroused and had to have sex with her.

I found the whole idea of another guy shagging my wife so exciting, I couldn't get the images out of my head. When I eventually told my wife this, she asked me 'would you like to see me doing it with somebody else?'  I said I would, and two weeks later I watched as a mutual male friend took down my willing wife's knickers and shagged her right in front of me. I must admit it was even more exciting than I'd imagined, and as soon as he'd come I was between her legs mixing my cum with his. A year on, it's become a regular thing....

My 48 year old wife confessed to me about being unfaithful 3 weeks after it happened.  She was out of town on a shopping trip and was staying in a hotel.  She walked over to a truck stop for a hot chocolate.  She ended up being walked back to her hotel by a 34 year old black guy.  He ended up in her room, fucked her bareback, got a blowjob and fucked her again.  I called her a nigger whore but, I was so aroused by her confession that we had sex right then.