And agreed, prison relationships (and I mean relationships, pure sex doesn't count) just expose hidden bisexuality. It's still not that accepted to be LGBT, so some people prefer to ignore their same-sex attraction, if they are also attracted to the opposite sex.
It does seem that in the real world, people who are bisexual often seem to come out as gay or hide their attraction to the opposite sex, because they perceive it as being more accepted to fall into one group, whereas admitting they fall into neither risks alienating both.
These two things, especially.
Conversely, it's interesting how soap operas and television often show characters (or perhaps it's just the writers) unable to reconcile gay or lesbian characters entering into hetero relationships. While same-sex relationships are now more commonly depicted in fiction, bisexuality is still often treated as being either non-existent or something that is "made up".
I think using something like a soap opera, or even some of these game characters/LIs, is not really the best example because the producer may have a character with known (read: observed) sexuality (A), and then introduce another character that doesn't match A's sexuality, but wants there to be a relationship between the two for dramatic and entertainment purposes.
The sexuality issue aside, both of the serious relationships that Willow entered into on the show were very formative for her character and both LIs became a part of that supernatural landscape (werewolf, witch); neither were "normal" people in that sense. Tara helped Willow develop her witchcraft further, since they were both into it, and we saw that with her (fridge) death, Willow became all corrupt, then was saved, and had more character development. Joss Whedon is known for having involved storylines that are developed in advance; he doesn't wing it. In that sense, if that was his plan for Tara all along, I think it makes the death even worse, since she was only put into the show as a vehicle for Willow's character development, and then it certainly falls in with the "bury your gays" trope.
There is also something to be said for showing a known gay character and then have them do something heterosexual. Apparently, in Empire (I don't watch the show), one of the main characters is gay and has been pretty open about it to the extent that it is a point of contention with his father. Last season, I think he made out with Alicia Keys or something and some viewers were, I think rightly, like, "Wtf?!"
(If someone watches the show and I got things wrong, my apologies. This is only what I've gleaned from bits on the internet.)
To me, that sends the message that the character isn't really, totally gay, because SEE hetero stuff!
In general, I think that Hollywood just doesn't know what to do with sexuality, ANY sexuality, as even heterosexuality is shown to be too rigid on occasion.