I'd be cheering him on. I figured out some time back that getting upset over such things is usually a one person issue - yours. If your boyfriend was upset about it, he wouldn't be doing it. If the straight guy was upset about it, he wouldn't be doing it. If you make a big deal out of it, everyone thinks that you're the jerk and you're outraged because you can't figure out why everyone else is against you.
If you stop and ask yourself a few questions, you might reconsider your angst. Is it really hurting anything or anyone? Is a little fun bad for you, or your relationship?
Another point to consider is that people like fun people. If you're the party pooper, you're the outcast. That goes for relationships too. If you go around putting up rules and restrictions and showing affront when the line gets crossed, you're driving a heavy duty bulldozer straight for your relationship.
These are my values and you may disagree entirely with them. I have however found that touch of levity and a reasoned response to such situations is much more well received than a pissing contest.
Did you consider joining him in licking the booze off then ending up getting lip locked in a big sexy kiss with your boyfriend?
It's another way to approach the situation. Why let them have all the fun. Get in the middle of it and make yourself more alluring to your boyfriend than the straight guy ;)
Your boyfriend and you won't be having a fight with neither of you getting any that night. The flip side is with a kiss like that, you're likely to make your boyfriend get all hot and bothered and you two might be having a hard time getting up for work after a night of romping with each other.