I've experienced this many times myself. Been vegetarian for almost 3 years now. Male co-workers make unsolicited comments towards it when they find out I'm vegetarian.
It's happened probably a dozen times or more. I am probably one of the most casual and tolerant non-meat eaters you'll meet. I'll chat openly with you about hunting, guns, best ways to cook bacon, steaks, burgers, etc.
I never preach or shame people for eating meat. Still, when guys find out, it's a lot of:
"You're a vegetarian? Ha, your girlfriend doesn't have to know, we'll let you eat meat here."
"No wonder you're always snacking, you only eat rabbit food!"
"Yeah, my wife tried that crap with me once, didn't work."
I even had a co-worker get in my face because he, "thought I said something about eating meat being unhealthy." I didn't say anything of the sort, but it was ironic coming from a guy who was pushing 350lbs and pounded 3-5 Mountain Dews a day.
Assumptions that I was only doing it because a woman was forcing me, or that I was implicitly shaming them for eating meat, or that I wasn't getting my nutrition, blah blah.