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.
"Heterosexism" is a broader term that includes "homophobia" as a form of "heterosexism".
I don't see this quite as an "unreasonable fear of homosexuality" but heterosexual relationships (and the players wanting to see this content) have been treated significantly better than homosexual ones (and those wanting them in game). Well, the latter hasn't been treated at all
So, it seems more like an 'erasure' type of thing going on which I think fits better into that broader category as it seems to stem not from fear but from not giving a crap (as long as there are heterosexual romances and people like them AND WE AREN'T LOSING MONEY over lack of SGRAs, whatevs).
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.
Yeah, I guess you could say it depends on how you look at it.
...heterosexist (or heterocentric, being that heterosexuality is the entire focus and that all things are secondary).
By "secondary" you mean "non-existent" XD