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...yes he is. (kidding)

He is, simply because there's no person in the world that would look good with him, except for Jounouchi and Yami no Yuugi.
Now shut your trap.

Alright then... Having braved Extended Discussion and sociology class, I'll take this on here.
Kaiba Seto is neither gay nor straight. In fact, he is anywhere from fifteen to nineteen depending on who you ask and what canon you mean. There are several reasons for this.
First, he doesn't really have much spare time to think about anything other than work, Mokuba, and beating Yugi (and in the anime case, school). With all that to deal with and sleeping factored in, he would have no spare time to deal with any romantic thoughts.
Second, even if he did, this is Kaiba Seto we're talking about, and he ranges from the evil mini antagonist of the dub anime to the not so nice loner of the Japanese anime's second series, to that scary sociopath pre-Death T in the manga, to an actual human being post Death T in the manga. Point is, his personality and people's understanding of it is fractured at best. That would naturally push people away. Plus no matter which canon you refer to, he does that on purpose too, with a constant three feet of space between him and the world. He simply doesn't believe he needs anyone else like that.
Finally, everything Kaiba-san has he's fought for tooth and nail. The idea that something could just come out of the blue and hit him is like the idea of luck vs. skill, and we all know what wins in his mind (skill).
So Kaiba Seto is an asexual person, not wanting anyone.

( xp ) Isn't that what I just said?

nicely put. okay, i have to answer i personally haven't a clue. but i agree with the answers luna gave.

I will continue to be the riged, stubborn, anti-yaoi fangirl you all have come to love yet hate, and protest that Kaiba Seto -sama is NOT gay, nor bi, but completely heterosexual. This does not mean I know exactly how he stands on the issue of other people's homo- or bisexuality, though...

Like Luna said, hes pretty asexual at this point, hes shown no liking for anyone in the show or out. Hell, if he met us I could bet money that he wouldnt fall for any of us. Sorry to crush dreams, but this is Seto, sure theres good in him, but he s not the type to fall in love with people, at least not now. Hes got his company and Mokuba to owrry about, he doesnt need any other boy or girl hanging on to him. Though I think he looks cute with Joey and of coarse..Mokuba, news flash, it isnt happening. Cause Seto is asexual to all those around him, anythign can happen yes, but its unlikly to say who Seto will fall in love with. He picks on Joey like a little boy would pick on a little girl, there a hint for Puppyshipping, I already made many rants of why for Tabloid(of coarse Mokuba would be older but thats not the point) and though I really dotn like or could say hate Prideshipping and I dotn see it happening, more like theyd be good friends. But they could happen, even if I dont like it. I could see hints between him and Serneity, Joey, Yami, Mokuba, cause they show chemistry. I dont care what gender they are, if theirs chemistry and possiblitiy of happening and of coarse, hints, then he could be with that person. Seto/Tristan, or Seto/Ryou, no interaction, no chemistry and no hints, so those are nos. And for anit-gay people, Seto/Mai never had interactions, why in hell would he date her? And though I agree that it was only a sign of human compassion when Seto only helped Bakura because of Serenity, I dont see them happening either. Its hard to see Seto with anyone, especially if your anit-gay and anti-incest. Mokuba and him have many hints and he is the only one Seto even gives a damn about in this world, but saying "their brothers' kinda elimiates that one, though we cant say for when Mokubas older but hell I know hes not gonna, but its more likely then some of the pairings people make. Mainly, Seto isnt interested in anyone around him, hes got enough on his plate, but we can dream so jsut deal with it. We can make him gay if we want to, you cant prove he isnt, we have reasons to why we thinks hes gay, you just thinkhes straight cause you cant see him like that, thats you thing, like how I cant see prideshipping while others can. Theres no proof, its our dreams, deal with it. You dont like it then dont look or read it. So im with you Luna 3nodding