Hm some people play different sixes, because they are intelligent and have a powerful imagination.
Intelligent in the meaning, that they went and asked women and men how they feel about their bodies and their sixuality.
Imaginativ in the meaning, they take the information from those people and try to imagine, how it would be if those feeling were their own.
If you're able to do that, then its extremly funny and interesting to play not only opposites sixes, but different character traits in all its facets.
How boring must it be otherwise?
So what character one chooses to play reflects their intelligence?

This is a
game. It's what people do with their free time. It's not a sensitivity training seminar.
I play mostly female characters, and it's not boring at all. Each one has something different to her. I find rp'ing a male less fun because it's less relatable to me, it takes a little more work to see that character as my avatar. I'll still usually play a male character in an RPG that has any replay value- I played a male in Oblivion and one in Dragon Age Origins- for the different story perspective. It's enjoyable in its own way, just not as much as playing a female lead.
Yeah that I still don't understand. The whole point of rping is, at least to me, to take on the role of someone that you're not. Do you and the other people that can't RP as a female also limit yourself strictly to humans? It should be easier to RP a human female than it would be to RP an Argonian, Trandoshan, or other completely alien species of either gender that doesn't have ~3.5 billion examples walking around on Earth.
I don't play beast races, no. Find them creepy. Elves are different, they are basically humans with a few different features and some different cultural traits.