It is perfectly acceptable to use leave time to attend a bachelor party. It's also perfectly acceptable to use it to go to a wedding, go to a beach resort, attend a comic book convention, go backpacking, visit your parents, stay at home and do nothing, or whatever you feel like doing. The point of vacation time is that you can do whatever you want, and you aren't "shirking" at all.
You aren't required to tell your coworkers exactly what you did if you don't want to either, but no one should look down on you attending a bachelor party. Though depending on what exactly happens at this bachelor party, make sure whatever stories you tell afterwards are work-appropriate.
Update to address comments:
I'm a new hire, and my paid time off is not yet available (it will kick in after 6 mos). At my workplace we customarily put "Out of Office - " as the subject line of the Outlook appointment notification sent to team members. [...] Yes, policy allows me to take time off and make it up beforehand/later. My question is just about apearances.
If you don't have PTO available to you yet, then how you get time off approved is dependent on the system at your work (and it sounds like you have that part figured out already). That doesn't change the fact that the bachelor party qualifies as vacation, and that specific activity shouldn't change how your time off is perceived.
For your Out-of-office message, I would probably just say "Vacation". This just says that you are not at work and discourages people from calling you, without actually saying precisely what you are doing and where you are. I would also do that regardless of what my vacation activity was, mainly because I value privacy and don't feel the need to broadcast to everyone what my vacation activities are. I'm happy to tell my coworkers about my vacations, but Carol from Accounting who I've never spoken to doesn't need to know those details of my personal life.
Now, there is the question of how taking vacation time early in your job would be perceived, regardless of where you are going. I would say that would depend a lot on your company culture and the personality of your boss. If they are willing to approve a PTO "loan" for you for a leisure trip, then I wouldn't worry about it.