If you want to avoid costume contest parties, good luck to ya!
The bar in this area with the most fun and least attitude - the latter in no short supply here - is Paradise Café in Cambridge. Age mix? Check. Go-go boys on weekends? Check. Overamped crappy dance music? Check. Porn loops playing silently on monitor screens? Check.
The "older" crowd, which in Boston is defined as 25+, favors Boston Eagle in the South End and The Alley near City Hall. What's left of Beantown's leather/"bear" scene gravitates to The Alley - and to Machine, which is near Fenway Park. Machine also includes a theatrical stage, where you can now catch a live production of "The Rocky Horror Show."
Gymbot posers and gayborhood guppies gravitate to the self-proclaimed sports bar Fritz in the South End. The gupsters also enjoy being seen at Club Café (aka Snub Café) near Boston Eagle and Fritz, as well as at D-Bar in Dorchester.
Although straight yuppies and affluent empty-nesters "discovered" the South End after "we" gentrified it, that section of town is still the reigning gayborhood. So it's often only a technicality whether you're at one of the actual Gay places or in any of the numerous trendy restaurants and watering holes that stretch for blocks along Washington, Tremont, and Columbus. The same holds true for some sections of Dorchester, particularly around the Ashmont terminal of the Red Line subway and D-Bar's territory in Savin Hill.
There are also a few places to go in more suburban areas. For a complete list (which would also include spots that "go Gay" on certain nights), pick up a free copy of The Improper Bostonian at racks and restaurants/bars all over town, or check out "our" newspaper at http://www.baywindows.com.