Because it's fun.
Yes, every girl is naked and always will be. Like rabid hairless rats those silly rascals. Pssshhh, girlss....
Girls aren't as "homophobic" as guys. It isn't that they sexually enjoy seeing each other naked, its just that they all have the same parts so its not a big deal to them to change clothes in front of...
I hoped you developed a relationship first!
Marvin Gaye and a nice dinner. Or just keep referencing your abnormally large penis, they'll want it.