I don't particularly see a difference in those two terms. Its just word play to try and avoid being offensive to the offenders. If it looks like a duck, walks like a duck, and quacks like a duck, it is very likely a duck.
That's really just a question of semantics. 'Heterosexism' (to me, at least) implies a concentration on heterosexuality as the be all, end all, to the detriment of other sexualities (as we have here, in SWTOR), whereas 'homophobia' (again, to me) means a definite action against someone or thing (action, idea etc.) that is or is perceived to be homosexual.
In this case, Bioware has made a definite action against SGRA's - which, by definition are an idea of homosexuality - but they have also crafted a world which is, in its entirety, heterosexual. I'd say that both terms apply to SWTOR; that Bioware's handling of same-sex content is discriminatory, therefore they are arguably handling this subject and content in a homophobic manner and that the game, as it is currently (and as it could possibly be after the introduction of SGRA's for companions if they do not also introduce outside [Flirt] options) is heterosexist (or heterocentric, being that heterosexuality is the entire focus and that all things are secondary).
Edit: To Stuffystuffs, I agree with you that it does feel like a form of erasure. It's a slightly unwieldy term, though, so I don't like to use it unless I'm sure that erasure is taking place. (Like if there had been lore from before release that said "X is bi/gay" or what-have-you and on release X was straight.)