I've argued Luigi's sexuality in a few forums but some people are doubtful. So I give you a fanart picture that I think says a lot about this topic *vomits*. And here is some insites from Kotaku.com:
Kotaku.com said:I believe we posted some this super-bizarre Mario fan comic a few months ago, with the leatherdaddy-lookin' Super Bros doing all sorts of running and jumping and climbing trees. But this particular pinup I had not seen until I did my daily GayGamer check.
"They have been through the tumbler, eh? So rough n' rugged. Personally, daddies don't do anything for me but I am sure some of you out there are thoroughly enjoying it. I think Luigi kinda likes being bound up. OH, and look at the 'shroom Daddy Mario is holding...a tad phallic?"
But seriously, look closely at Luigi's g-strung crotch. It's flatter than a mannequin. Is this some extra layer of kink, or did the artist just turn his nose up at rendering the bulge? The world may never know.