Finding other people attractive is done by our unconscious mind. It’s natural. When someone who you find very attractive makes direct eye contact with you, you get a kick. Dopamine is released. It makes you feel good. Enjoy it.
Now your conscious mind can tell you, it would be interesting to know this person better and have a good conversation. We can know many people without pursuing sexual relationships, especially when your existing long-term relationship is stable and makes you happy.
Flirting on Fluther is harmless as well.