Because nudity is akin to sex and sexual ways, which are taboo in many societies. Upstanding citizens do not have nudes, in general. Especially women or nudes hinting at same-sex romance.
It isn't right, but it is.
Edit: I'm not saying I agree with this. But it doesn't take much to see folks putting others down for nudity. YMMV depending on where you live in the US. There is a reason most politicians (in the US) wouldn't get caught with nudes and I'm guessing that in some areas of the world, it would be even more detrimental to your life. It is the same line of thinking that punishes women for being "sluts" but are OK with men having a series of one night stands.