What Quazi said. I'd hate it if my girlfriend took up gaming - though I've tried to get her to play some solo games with me. I wouldn't mind if she joined one of my weekly games, but if she was in both, I'd be a little miffed.
Part of it is her personality (I think she'd take my DM "attacks" on her character personally, plus she's very competitive - which isn't good in a co-operative game), and part of it is the fact that we spend a lot of time together. Gaming is one of my escapes - sort of "me time" with other people involved. The rest of the time (when I'm not at work or at schol) I'm with her. I'll keep her away from gaming.
Regarding "guys night out" - I don't associate it with D&D, simply because both of my groups have a gender mix. In fact, most of my groups have been mixed gender; in high school, I even DMed for an all-female group (sadly, no "Dear penthouse" moments....). I've run all-male groups, but it's never been something I've consciously picked up on. Really, I don't see D&D as a "guys night out game".
When I want "Guys night out", I turn on the TV and we watch hockey. Last year, we had a fantasy hockey pool (what my girlfriend's dad describes as "Dungeons and Dragons for people who had girlfriends in high school") that often turned into a very competetive Guys night out. It was fun cheering for San Jose for the most part, but going absolutely crazy when Henrik Sedin scored a goal on SJ simply because you have him on your roster - a group of guys cheering for different segments of both teams is definitely a fun dynamic... especially when there's booze involved.
So, yeah - I like "guys night out", but not when it involves D&D.