Alright, we're going to get a bit technical here.
None of the characters' sexualities are shown at any point in the series. In fact, most people assume they are gay/lesbian because of shipping. None of the characters have any interest for one another, never have sex, and so it is not canon.
We can only sort of, I could say, "confirm" Rarity's sexuality however, as well as Spike's. However, because Spike is male, I would imagine that the intended sexuality for the show would be heterosexual. Although I can't confirm that this is canon with every character, I would imagine it to be that way.
(Also, I don't mean to be homophobic here, but I'm going to say that heterosexual people are more accepted than homosexual people. If her sexuality went canon and she was gay, parents would most likely get angry, saying "you're influencing my child to be gay" or something. I'm sorry, but it's true, or at least it's another reason. I wish it wasn't.)
However, going by opinion, I truly don't care.
(And, here's another one, or at least a theory. I highly doubt Hasbro would make them marry a random stallion. They'd have to introduce them, first. So now, we're leaning both ways; heterosexual and homosexual. Who knows, they might just be bi.)
Now, there's one thing that go along with my theories, as well as everyone else's theory of Rainbow Dash being gay; I can't say that they're true, nor can I say that they're false.