Are the two meant to stay together? That’s kind of important, imo.
I think you build the sympathy for the woman through story. Show that she wants and needs a lustful affair and then it doesn’t matter too much what he is like, your reader would want her to get with him.
An example— D.H. Lawrence’s Lady Chatterley’s Lover which is still the most erotic book ever written in my opinion. She had a mental connection with her husband (her husband is in a wheel chair if you don’t know the story) but there was no physical connection and she was sex-starved, affection-starved, so much so that the MC describes their first copulation as one of pure sympathy for her. He describes love as physical sympathy. You wanted her sexual needs to be seen and met by her lover. And they were.
So here comes why I ask if they should stay together. It would be a huge betrayal to have all her needs truly met and satisfied and then have him discard her. That could wrench your reader’s guts out— if you want to as a writer. If it’s going to just be a fling then have him not satisfy all of her needs and somehow leave her wanting, emotionally, intellectually, or physically and you can play around with her still trying to make it work and eventually growing up and learning what she really needs and moving on, maybe even finding her needs met elsewhere.
To DH Lawrence it seemed like his argument was that a good physical sexual connection WAS love, but he also showed emotional and mental connection between the two characters no matter how he framed it in his mind.
I guess just make sure the physical need is shown on her part and depending on your story show her satisfaction on whatever level: Passion, intensity (it can be from frequency, sacrifices made to meet, fantastic and multiple Os, foreplay, banter, etc.
I guess I could go on about things a majority of women usually like as qualities, but most people know them: tall, good-looking, strong, smart, capable in multiple areas and maybe vulnerable in one or two only she knows about and can help him with, money and/or adventure doesn’t hurt— yada yada. I also personally like perceptive, intelligent, interactive, kind and funny and I will take those things over looks any day. We all love intense sincere interaction, feeling really seen, really appreciated, and understood and our needs met. We want to see that in books. Honestly with the way women’s sexuality works it’s about making sure to write that he finds her beautiful, intelligent, witty, and basically gets boners constantly from her— this is true. We will often love any respectful (of our choices and autonomy) man who adores us and who we turn on. We even want to hear that another member of our sex is found to be super beautiful and turns men and her man on constantly. There is a narcissism for our own sex’s beauty and sexual power there. Oh, it’s so true!
I had more to say than I thought. Good luck!