Employment is a financial and time relationship between an employee and an employer. Just like other relationships that involve money and time, participating in activities that could impact or even end your current relationship need clarity on expectations and relationship longevity.
If you are in a relationship with someone you may want them to always look their best regardless of how that may attract others to them. If you expect them to only look good in private, that arrangement may also work. But if your "special someone" goes out "for a night on the town" without you frequently, maybe you should have different expectations than someone that doesn't, if for no other reason than the exposure to opportunities.
So, if your employer expects complete loyalty from you (for whatever reason), then you should not participate in these activities. If your employer doesn't mind that you "show off your talent" or even encourages it, then other things are clearly more important than loyalty and relationship longevity.
Some employment relationships imply loyalty, like being a significant shareholder of the company, "key person" or founder. Additionally, remember that managers that advertise and participate in "employment seeking" activities should expect their subordinates to do the same. It sends a clear message that their time is well spent looking for better places to work.
It should be well understood that relationship longevity and expectations should be impacted. Employers will have a varied tolerances and responses to your behavior, but you can't expect the "really good ones" to not care. But then again if you worked for one, you probably wouldn't spend time "keeping your options open" because you'd be dedicated to your current employer, just like any really good relationship worth keeping. (I'm assuming career guidance, the job market, etc. are not in your line of work, of course.)