I can maybe see how people might view this car or that car as a girly car, but I can't for the life of me understand the logic behind the V6 Mustang being a "girly" car. So if you like the Mustang and you try it and then come to the conclusion that you don't really need 2 more cylinders under the hood + you don't want to pay more (higher insurance, higher price, less economy) you all the sudden lose your testicles? And aren't most Mustang sales V6 anyways?
There is just no way I can view a guy for instance driving a 3.6L 5-speed manual Mustang as a girl. As I had to make the same choice when I was looking at the CTS and the CTS-V and the CTS came out as the logical choice because I didn't need the extra power, I didn't want to pay more for insurance, I didn't want to pay more for gas just to say I have a V8 under the hood when the power of the V6 was good enough.
It's more like the Mustang starts out being girly, but if you get the V8, you gain testicles. For the CTS, the V6 may be good enough. But the V6 Mustang isn't good enough. Hasn't been, at least. The CTS (and especially the -V) is a different class of car anyway. The CTS is the "normal" version of the CTS; the -V model is considered a bit extreme. The Mustang GT, however, is what should be considered the "normal" Mustang, with the V-6 version a step down. The normal CTS is a fine automobile. So is, for example, a BMW 5-series; nobody's going to slam a guy for not stepping up to the M5. Same with, say, the Honda Civic. You don't have to get the Si. The base Civic is pretty good on its own. But the V-6 Mustang has all the downsides of the V-8 Mustang (big, heavy) without the upsides (fast).
But it also has to do with the styling of the Mustang in the 80's-2000's, for me at least. It was... just kind of girly looking. In a way that the Camaro and Firebird of the era were not. The Mustang was a "cute" car. Camaros and Firebirds were more "bitchin'". The Mustang was not a small car, but it had that impression, especially in comparison with the F-bodies. Not as long, not as low... nobody would ever try to compare a '97 Mustang with the Batmobile. The Mustang just didn't look very aggressive. I don't think I know a single girl who drove a Camaro or Firebird... but I knew plenty who drove Mustangs.
Anyway, this isn't to say that it still holds for the newer models. The 2005 redesign of the Mustangs returned a lot of the good old fashioned aggressive look.
TL;DR version: the previous generation Mustang V-6 was "All show, no go" -- and the "show" was girly.