confused about honesty and trust

hey everyone, i need some help figuring something out. i work nights at the local diner and it's been crazy busy lately with everyone going back to school or work after break. one of my regular customers is this really nice guy named mike who comes in around 1am every night for coffee and dessert. we talk a lot about life, movies, books - all sorts of stuff. i started dating jake about six months ago and it's an open relationship but i'm not feeling very adventurous right now. anyway, last week after mike left the diner, he texted me asking if i wanted to get coffee during the day when i have mornings off. i said no thanks because of my boyfriend jake. a few days later i got another text from him, saying 'i know we're friends but are you interested in more?' and i told him honestly that it was complicated right now with my relationship situation. but then i started thinking about why he asked. did i give off signals? do guys like me think differently than what i thought? so here's the thing - today after work mike invited me to a movie during his shift at another diner where we could talk more and catch up. i really wanted to go out for drinks but didn't want to be disloyal, so i ended up going to get coffee there instead. that's all fine except it made me wonder if being friends is okay when he's asking about something deeper. i feel bad like i might've hurt his feelings by not wanting to do more right now and i'm unsure how to handle this in an honest way without messing up my relationship with jake. what would you guys do? am i the asshole for keeping things casual despite obvious interest?