i think most of the reason Alois acts the way he does toward Ciel, has to do with jealousy.
He's envious of Ciel and wants more or less to destroy him, but he's immature and impatient. The only way of knowing how to get close to someone has been learned by being suggestive and lude.
I do think he is gay. But it might have more to with spite and jealousy. He's trying to take away everything from Ciel since apparently there can only be one rich, orphaned, traumatized brat with a demonic butler.
Baron Kelvin (Pedophile, so it's a given).
Aleister Chamber (hits on him every time he sees him in the manga).
Undertaker (heavily & repeatedly suggested).
Soma (guy is just a little too friendly & too clingy).
Lau (always trying to dress Ciel in drag).
Edgar Redmond (like his uncle Aleister, but less eccentric).
Nina Hopkins (openly attracted to both Ciel & Marron).
Alois & Claude (because Black Butler II is straight-up BL).
Lizzy Midford (Even if she is his aunt's daughter).
Sieglinde Sullivan (She molested him & he didn't like it one bit).
I took Sebastian off the list long ago. Sebastian is simply not interested. He even burned his glove after touching Beast. The nun is the anime is canon to the anime only. He finds Marron annoying. He touches Ciel quite a lot, but he really doesn't seem to like him very much.
Ciel is naive as shit when it comes to sexual tension. He didn't understand what Alois wanted & he didn't get Sieglinde's intentions no matter how many times she tried to explain it. He doesn't like being touched by anybody, but allows Sebastian to anyway. Undertaker freaks him out in a similar manner to Nina, Kelvin, & Aleister. He does like Lizzy, but like with Soma, he doesn't like it when she clings to him. He surely knows Sebastian will take his soul before he can marry his cousin.
As for Alois...He is basically Gilbert Cocteau from Kaze to Ki no Uta. Toboso LOVES referencing other movies in this series. The most blatant ones were Titanic & Harry Potter (manga only).
Lastly, it should be known that prior to Black Butler, all of Toboso's other mangas have been BL, hardcore yaoi, & shotacon...Almost always with tragic endings.
I think that Alois is extremely lonely... And he will do anything to find someone who cares for him. But the only way he knows how to get close to anyone ( in this case Ciel) is to seem to suggestive, like gkazeuishida said, because the only form of love he was ever given was sexual attraction.
Alois Trancy isn't gay. He wants to have Ciel only to kill him in front of Sebastian and they both know this. The thing is Ciel challenged him to a duel, during the duel Ciel said to Sebastian and Claud, "stay back until I kill him." So he isn't gay he wants to kill him.