I think Luffy is gay. Before you start ripping on me for saying such thing in bad taste, yes. I am, in fact, dead serious. You look back and there were only two instances when Luffy appeared straight and both times were very brief and for comic relief (The Mr. 2 turning into Nami and showing 'anatomical correctness' in his ability and the Happiness Punch in the showers at the end of the same arc.) Now, he seems to bond well with guys more than he does girls as he seems to interact with them more in the series, especially Zoro, Usopp, Franky, and Brook. Anyway, Zoro is Luffy's first mate and Usopp is his best friend, so I neither really count as much. However, the more and more I look into it, other than the two times I mentioned. A good example is of when Luffy dropped in on Hancock, who was stark naked taking her bath. That being said, even the iron-willed Vice-Admiral Momonga would have succumbed to lust if he didn't stab his own hand.
Also, there have been countless other times when he was with Hancock where he should have at least looked like he had a SLIGHT interest in her. But alas, not only did he not show it, but he was immune to Hancock's attack.
Now that I have argued about Luffy not being straight, I'll argue about him being homosexual. First off, he is good friends with Mr. 2 and refers to him as 'Bon-chan'. My second and final point, because I may need to review the series again to get another point, Dumbledore was also gay and ne never displayed this. Well, in any of the books I read, anyway. This shows that Luffy could have a level of self-control by just being eccentric in general.
Anyway, this is just a theory, personally.