I guess this is a semantics discussion, without any real right or wrong answers. What I'd shoot for is consistency. What do you call other bikes, intended for other riders? For me, I think it's boys' bikes, girls' bikes, men's bikes, so it'd be ladies' bikes. In all cases, we're talkin'bout plural & possessive. If you think of it in terms of boys bikes, girls bikes, men bikes, then I guess it could be ladies bikes... but you know that you've never, in your life, heard anyone who speaks English fluently say "men bike".
In practice, I generally avoid this stuff, as it makes more sense (to me, anyway), to talk about bikes in terms of wheel size (20", 26", 700c, whatever) and I'll describe the the ladies' versions as "step-thru", if it comes up. I know too many guys who ride step-thru bikes, dues to physical limitations or medical conditions. Plus I know a lot of ladies who ride diamond frames or "men's" cantilever-style bikes b/c they're the radder choice.
The differences between the terms mentioned in this thread are something that most wouldn't notice in casual face-to-face conversation. On these forums, we tend to have pictures, but even if we're just saying like "dude had some old Schwinns for sale cheap at his yard sale, but I passed on most of them b/c they were girls' bikes", no one is going to go on a pedantic rant about it. (But yeah, I'd probably just say "dude had some old Schwinns for sale cheap at his yard sale, but I passed on most of them b/c they were all step-thru". And, let's be honest; step-thru ain't spelled right, neither.)
In the end, we're talking about frame geometry, not gender. It's not like you can take your bike to the veterinarian so he can crack open the bottom bracket, count the number of ball bearings, and tell you "it's a female." We're talking about steel, here. And yes, to be sure, I'm aware that historically, step-thru bikes were marketed towards women... but "these days", a lot of conventional diamond frames are being offered in women's geometry (basically, shorter top tube, some change in component spec, and a pastel paint job) and that's been going on since at least the 80s.
That's a pretty long post. Sorry.
