Boundaries: Should I ask him out?

For about a year now, I've had an intense crush on one of the instructors at my gym, and it's not going away (though at least I can talk to him without giggling now). He, my husband, and I are all late-30s and on friendly terms, hung out in a group with a couple of my crush's friends once, and the instructor has twice given me free equipment to train with and rides home a couple of times, so I'd say we get along. He's flirted with me a bit, but he does flirt with every woman in the gym. I know he's single and kind of a fuckboy, which is fine with me since I don't want a relationship anyway so much as a fling. I don't think he knows my husband and I are open.

I don't want to make things awkward at my gym though, as it's a small gym dedicated to a particular sport and there's not another gym I could switch to if things got weird. I am totally capable of banging other people once or twice and remaining friends (if we stay in touch at all), but in my experience some dudes get a lot more weird about that.

I fantasize about this guy CONSTANTLY. I kinda need to fuck this guy raw before I implode. But I also need my gym. What do you think? Seize the moment and ask him out, or just keep on dreaming and stay on professional/friendly terms?