Anon tries to understand his coworker

I'll never understand how difficult people make things by not being forthright and making things awkward:

"Is it okay if I bring my boyfriend?"

"...I think there's been a misunderstanding. I had asked you on a hike as a fun first date, I didn't realize you had a boyfriend. I'm going to bow out of this, but I can give you the hike info if you want to take him." And then you laugh about the misunderstanding the next work day, and keep things at work from then on. No reason for it to get bad. Flirting is still fun! Just leave it at that without expecting anything more.