Firstly, sex and gender are two entirely different concepts. "Sex" refers to the reproductive organs/characteristics that you were born with: male vs. female. "Gender", by contrast, is constructed socially, not biologically: it refers to the roles, behavior s and attitudes that a group of people consider 'appropriate' for masculinity/femininity. And in many ways, these 'rules' are completely arbitrary.
In some cultures, such as Sweden, a man painting his toenails would be seen as completely normal. Sweden is known for being quite gender-neutral, so such a thing would be more socially acceptable there. The same would hold true for Taiwan, where it's common for some men to dress as women in cases where their aren't enough daughters in the family. This is a long-standing cultural tradition, and has served Taiwanese society well over the years.
By contrast, your profile says that you live in Texas, which is traditionally very conservative. The gender roles in Texas tend to be much more strictly defined, so something like this probably (and understandably!) seems very odd to you, because it isn't what you're used to.
The thing to remember is that just because you percieve something to be 'wrong', it doesn't mean that it is (or that the rest of the world sees it the same way). Gender roles are completely, 100% socially constructed, so they will inevitably be different depending on where in the world you are, and which specific group of people you associate with.
Of course, it's perfectly okay to have preferences! But if men want to paint their toenails, why not let them? It isn't hurting anyone, and you're free to ignore it if it irritates you. :) Ultimately, these people have grown up in an entirely different setting than you, but that doesn't make their attitudes and behavior s any less valid.
To answer your second question: yes, I'd be okay with having a boyfriend who painted his toenails. It wouldn't really attract me, but I definitely wouldn't be turned-off by it! Even though that kind of behavior isn't exactly common where I live (in Canada), I've been lucky enough to be exposed to a lot of different cultures and definitions of gender, both of which have helped to broaden my own. I would suggest doing some online research looking into gender roles, and how they vary across cultures. It'll probably seem weird at first, but you might be surprised by how normal it all starts to seem after a while. :)