Moonlighting as a Licensed Surveyor — No Written Policy, but Employer Objects

I asked my manager about occasionally performing small residential boundary surveys independently on weekends. He strongly opposed it, compared moonlighting to "cheating on your wife," and said it would be disloyal to a company providing good benefits.

However, there is no moonlighting restriction in my offer letter or employee handbook. Another employee also owns an LLC, although I'm not sure whether they actively operate it.

My proposed work would be completely separate from my employer's business, using my own equipment, software, insurance, and professional authorization. No company clients, resources, or working hours would be involved.

Has anyone experienced something similar? Can an employer prohibit independent, noncompeting work based solely on a verbal objection? Would you recommend consulting an employment attorney before proceeding?