So, 21/22, graduate trainee... How it sounds anyway.
If you want to get to know colleagues as colleagues you go out for lunch or straight after work. And you make it clear you're not single.
Come off it with the "couldn't say no" stuff. I'm not sure why you think declining an invitation is rude but letting somebody think they're going on a date with you is somehow not rude. You're either being deliberately unfair, or letting people walk all over you.
Would your work send you on an assertiveness course? Then you can learn how to say no without feeling rude or being rude, and not get yourself into situations like this in future.
It's a bit of a passive aggressive cop out to only clarify you have a boyfriend by telling him you want to meet near the station. Don't you like him enough to be direct?
You're the one who didn't bother telling him you had a boyfriend before accepting his invitation to go on a date, so you're the one who should have to deal with the discomfort of clearing things up - rather than leaving him to feel stupid and embarrassed because you've brought up your boyfriend in such a passive aggressive way as to suggest this colleague should somehow have already known about your secret boyfriend.
It's not a great way to behave with somebody you need to have a decent professional relationship with.