The sheer act of having a drink bought for you carries an automatic, inseparable component of there being something more to the situation than just you and the guy getting to know each other on a platonic level, even if you state up front that you have a boyfriend and this is just platonic.
A guy does not buy a drink for a girl just to have a new platonic friend. It is insincere and disingenuous to think so. There is always something more in his decision. After all, why doesn't he buy drinks for other random guys to make more guy friends?
No man approaches a stranger out-of-the-blue in a bar to offer to buy them a drink, having no prior interaction or possible motive, unless they are interested in having sex with or dating that person.
You boyfriend may be ok with you accepting drinks and chatting with new men at the bar (though part of me suspects he's trying to play it 'cool' and not seem like the controlling type), but even so it is a bit unfair to the men buying you the drinks knowing what their intention is.
But EVEN if you disagree with that, consider that whatever friendship blossoms from this social exchange will likely be surface-level or completely false. The guy, your newfound platonic buddy, had the intention of getting in your pants. You politely accepted the drink but made it clear that you have a boyfriend. "That's fine, I just want to chat and meet a new friend" he replies, lying through his teeth. From that point on he either makes further attempts at hitting on you, gives up and never talks to you again, or maintains the charade, perhaps even staying as a regular 'friend' in your social life. But in the back of his mind he will have that true intention and desire.
Friendships are built on a foundation of shared interests and activities with each other, not a free drink bought by someone who already had romantic intent in his mind.
Join a meetup group if you want to meet new platonic friends and don't try to walk this gray, have-your-cake-and-eat-it-too, area when it comes to accepting free drinks.