I'm going to date myself with this reference, but: Warning, warning, danger, danger Will Robinson!! (Google it)
A crush is normal. But you are heading into dangerous territory. Boy, I wish I could talk to you for a while. When I was in my 20's, many decades ago, I was an idiot. But at least I was an idiot without kids. I did things that make me cringe now. I had some poor parenting, if that's any excuse, but still, I know all about these things like crushes and flirting and affairs, from both sides of the whole sordid thing, because I did things I shouldn't have, and they all began the way you are describing.
1 - No long-term relationship can stand up to the allure and excitement of flirting with someone new, so don't put your marriage to that test.
2 - Let's just say you allowed yourself to have an affair with your crush and you left your husband for him, -- in a few years he would be the old boring guy your husband is now. The excitement of a new relationship DOES NOT LAST. Anyone becomes boring to an extent after 9 years.
3 - Probably most important: You have kids. You chose to have them. You cannot break up their family for what will ultimately prove to be fleeting. If you and their father break up or have major problems your children will be harmed. You owe it to your children to keep their family intact.
4 - A little flirtation is one thing -- as someone below said, we don't grow blind and deaf just because we are married -- but if this flirtation looks like this might become anything more you MUST remove yourself from this situation, even if it means quitting the gym. There has to be a line that you just don't cross, no matter what.
Everyone else gave you good advice. I'm telling you from experience that if you pursue this it will end in disaster, and harm your kids. Don't do it.