It's normal-ish! I am very susceptible to it when I am feeling a bit down. I have learned to separate these feelings from those of my xH and now my DP. It's just me fantasising and has no basis in reality. It's the way I am, I used to love meeting eyes with a sexy man across a crowded room, etc. Can't help it, the thrill of the chase, the thought of it is so exciting. But I have never acted on it and this is why...
A close friend of my mother's (aged 33 at the time) harboured a crush on one of the builders carrying out extensive work on her house. She was beautiful and wealthy, he was 21 and muscly, there was a lot of chemistry between them and the crush became flirting and then something more. He used to come and stay when her DH was away working, and she fell in 'love'. Inevitably, the builders left. She was heartbroken, as she'd become unhappy and lonely in her marriage and thought she'd met 'the one'. She was obsessed with him. He was everything she'd dreamed of, young, good-looking, funny, caring, fantastic in bed... Whereas DH was older, ran a business, worked long hours, and was often tired and distracted.
So, the marriage ended, and she thought she could pick up again with the builder and live happily ever after. She met his friends, who all had very young girlfriends and rode mopeds. He was happy to eat at cafes, she wanted to go to restaurants. He was happy to go to a pub and spend the evening playing fruit machines, she wanted to go to the theatre. The sex was incredible but their lives and interests were so different it quickly became clear there was no future in it. Gradually she realised her DH had been working so hard to provide for her - she didn't have to work - and he loved her so much he pushed himself further and further into his work so he could afford to pay for the gorgeous house, the holidays and amazing cars. THAT was the problem, and they could have fixed it.
Sadly, it was too late for them. She's bored us all endlessly with how she threw it all way in the name of lust. But it's a good lesson for me, because I can see that a fantasy is just that - a fantasy. And what is around me is real, and safe, and I really really love my DP and know he is perfect for me.
Sorry to go on, I need to re-affirm it to myself sometimes!!