From a cis/straight guy's perspective, I used to find myself in this situation a regularly. I've found a good combination of pragmatic coping strategies and reframing the person is helpful. Others have mentioned common coping strategies like distraction and minimizing contact, but I find this alone doesn't help set you up for healthy relationships because it only treats the person you're infatuated with as the "forbidden/unworthy object of desire" rather than their own person. So a lot of the reframing I'd do was geared towards reframing then as anyone else that you're not interested in pursuing, so things like "I'm not their person and they're not my person", "they're not into me and that's not hot", "they're a friend/sibling who has their own life, I'll support them in theirs, but I should focus on mine"
It's also always good to put yourself in their shoes. If you were in a relationship and knew someone who you're not into was infatuated with you, how would you want them to act? I doubt you'd want them to pursue you or conversely treat you like toxic waste just to cope.
It's infatuation, it will pass. Love must be mutual and is built over time.
TL;DR: As a guy, this usually ends up being a lot of "Don't be a creep, just be fucking normal."
Ug. Unrequited love. I think they wrote a few plays about that back in the day. They usually had bad endings.
Humour aside, the best way you can deal with this is tackle it head on. He's Just a Guy, honestly, and there are many other fish in the sea. Put yourself in his SO's shoes and ask how you'd feel if the person you were in to was macked on by somebody else. Then imagine him doing all your pet peeves: leaving the toilet seat up if you're a gal, hogging the covers on the bed, letting the dog or cat out, overfeeding or underfeeding said cat or dog, and 101 other annoyances I almost GUARANTEE you he'll do at least one of, if not far more.
As for him being able to tell, what you do about it depends on what your and his actual relationship are. For instance, if you're mere acquaintances, avoiding him is probably the best bet. If you have a professional relationship with him, keep it professional and channel your feelings for him elsewhere, preferably something healthy like meeting new people and expanding your after-work friend group. If you're friends with the guy, and you have a strong friendship that you don't want to lose, you'll have to evaluate that friendship to determine if you should keep things bottled up and do the whole 'look for dates' thing, or if you level with him and tell him you have the hots for him but understand those hots will never be acted on, or something else, and see if you and he can work through diffusing this. Humour is my go-to for these kind of solutions. "Hey, I really like you, like more than friends, but I know you are dating so and so. Can you give me your top ten reasons why I shouldn't date you? Be graphic!"
It's tough in love out there, and I wish you the best of luck!
There are many general guides regarding getting over unwanted infatuation, f.e. https://www.wikihow.com/Get-over-Someone-Who-You-Know-You-Can't-Date
Reading those could help you reorganize your thoughts.Personally I found that it can be easier to work through stuff if I try to imagine how I would help a friend in my own situation. That helps with getting perspective.
