So I had an idea for a list the other day relating to superheroes who have the words "boy", "girl", "lad", or "lass" in them (mostly inspired by the Legion of Super-Heroes), and I began compiling the list. What I found led to an interesting conclusion: DC comics has MANY more characters with boy or lad in the name than Marvel( Aqualad, Matter-Eater Lad, Colossal Boy, etc), and Marvel, in relation to the number of young male characters, is much more comfortable tacking girl on the end of a character's name. Out of extant, well-known characters, I couldn't find a single example of a "boy" named character, but I found MANY young females with the title "girl" attached to their name. Spider-Girl, Thor Girl, U-Go Girl, No-Girl, Gorilla Girl to name a few.  
 
As I began to ponder this, several conclusion arose: either Marvel doesn't want to portray men as anything other than the masculine elite (being men and not boys as that somehow degrades them) or Marvel is more comfortable with keeping women in roles that don't allow them to grow up. Women don't need to exert power, especially over men, and so must remain girls.  
 
Now I'll be the first to admit that I am a feminist. I'll also be the first to admit that there are probably more than two solutions to the problem I've shown above. However, I do think it odd that DC seems to be equal in the number of boy/girl characters it has, while Marvel is strongly one-sided. Is Marvel sexist? To be fair, I should also point out that the "lady" characters are heavy in Marvel as well while the "misters" are in short supply (while "doctor" is all the rage). What does this say about the two companies?

Cool topic, DC's characters tend to age, whereas Marvel never seems to. Take Tim Drake for instance, he started a young kid who now appears to be at least 18. On the other end is Jubilee--introduced as 14-15 and is now probably 16 or 17,
 
Also Marvel seems to not favor attaching girl or boy to that many characters in the first place, probably because DC does it so much, and there is a lot of strong feminine characters in Marvel, I would say MORE so than DC

The problem with the examples you mentioned ( Spider-Girl, Thor Girl, U-Go Girl, No-Girl, Gorilla Girl) is that they are all teenage characters that are relatively new to the comic scene.    If any of these characters stick around long enough they will probably get renamed.  Look at Invisible Woman, she used to be called Invisible Girl.  It's the same with Jean Grey, she used to be Marvel Girl.

Invisible Woman trumps you. 
 
Meanwhile Legion of Super Heroes are all girls, lads, and whatnot. 
 
fail

I think any name ending in man, woman, girl, boy, lad, lass, etc. sounds incredibly stupid. 
 
Just my two cents.

I think it would be a big stretch to call it sexist. That presumes that many of Marvel's female characters are held down with demeaning "-Girl" names, when most aren't and those that have those names are young characters where the name is appropriate to their age. Sue Richards grew to use the name Invisible Woman. Jean Grey grew and dropped the name Marvel Girl. Jessica Drew was always Spider-Woman. And many female characters just don't use a gender-based naming convention, or if they do, it's something like Ms. or She-. The few examples you give are mostly on the obscure side too. You don't really see this much with the more prominent characters. Cassie Lang is Stature rather than the more obvious Ant-Girl. Nico Minoru is Sister Grimm rather than something like Witch Girl. X-23 is... X=23.

Eh, another possible problem with some of your examples, are some are that way for ironic reasons (Tarene is an alien who doesn't quite get Earth culture and so out of her limited understanding and admiration and respect for Thor, she took quite a camp and cheesy her name) (No Girl has no body. She is a brain, she sort of pokes fun at the use of the whole name thing, her best friend has the body of an 8 year old but looks 80, her name is Ernst, I mean...?!?!), and some for branding reasons (Spider-girl = money ideally) (as well as their already being a Spider-women... actually a number of them) (as well as a lot of characters referring to her as Anya), and I mean, I know you acknowledged DC's use of the word boy, but DC also has, Power Girl, Saturn Girl, Wonder Girl, Phantom Girl, Duplicate Girl, Dream, Elasti Girl, Sensor Girl, Night Girl etc etc, I could go on, if anything, Marvel has the opposite problem. If we look at X-Books, at each new generation of students, its males that usually never quite make it past a certain mark, degree of characterization or somehow they tend to suffer from regression. Generation X is probably the most obvious example. (this is strictly as it applies to younger heroes) Anyway, so yeah, in terms of actual storytelling? Plots and what characters they are based around? Point of view characters? Slanted towards benefiting female characters, as far as names? Ignoring the fact that a lot of writers try to be too clever or a bit of a smartass and then trying to actually substantiate that women is better to use than girl, when there is nothing wrong with girl, I mean... maybe? What's more important though? Plus this is comics. Marvel and DC, don't want characters to age, because most fans don't want characters to age. (or at least this is an idea that tends to be supported to some degree with sales)  
 
Anyway, this almost seems like 'punishing' Marvel for given almost a priority towards younger female characters. (which I could explain in greater depth) (its a negligible priority as well, so its not that I think they do males well, or that DC doesn't give fair chances for their younger characters) 

When Jean Grey emerged from the cocoon that the Phoenix had placed her in, she didn' believe that the Fantastic Four really were who they claimed to be... One of the reasons was because Sue Richards called herself Invisible Woman as she no longer went by the name Invisible Girl.

Jean herself used to be Marvel Girl and abandoned the immature name on two different occasions.

There name "Spider-Girl" still exists because there is already a Spider-Woman.

It seems to me that Marvel's ladies are more mature than all the Lads & Lasses of the DC Universe.

Cool topic, DC's characters tend to age, whereas Marvel never seems to. Take Tim Drake for instance, he started a young kid who now appears to be at least 18. On the other end is Jubilee--introduced as 14-15 and is now probably 16 or 17,  Also Marvel seems to not favor attaching girl or boy to that many characters in the first place, probably because DC does it so much, and there is a lot of strong feminine characters in Marvel, I would say MORE so than DC

@Blurred View: 
I hadn't really thought about longevity as a part of the equation. Of course, I knew about the name changes for Sue and Jean (I didn't want to over-complicate my argument by poking holes in it) but hadn't seen any modern examples in a quite some time. However, today I learned that the Alpha Flight character Purple Girl was just renamed the Purple Woman in the new Alpha Flight series. Her character has been around for about 30 years, so I guess it's about time she becomes a woman! 
 
@xerox-kitty: I would also agree with you that, on the whole, Marvel women tend to be stronger than DC women and more fleshed out as characters. I guess that's why I'm disappointed in this little set-back. 

Marvel doesn't have all that many female characters whose names end in Girl maybe like 10 where as Dc has Supergirl, Power Girl Saturn Girl, Lightning Lass (lass means girl), Bat girl, wonder girl, and pretty much every other female member of the legion of Super Heroes so the OP was actually highly inaccurate if anything DC is more at fault for this than Marvel  

You're kind of stacking the deck, at DC, by bringing in the Legion of Super-Heroes. That series could almost be considered to occupy it's own separate continuity from the larger DC Universe. Without their inclusion, DC's numbers go way down. Aqua "Lad", Super "boy", & "Kid" Flash, on the male side. Bat "Girl", Super "Girl", Star "Girl" & Wonder "Girl", on the female side. Admittedly, Marvel does seem to shy away from using the young male identifiers in code names. Marvel "Boy" (of which there were three) & Iron "Lad" are the only ones that readily jump to mind. I think the "man", "woman", "boy", "girl" identifier has largely fallen out fashion. In general, new characters tend to carry one word names that are descriptive of different aspects of the character.