Because I never want any kind of media--especially one that has built a reputation on inclusion--to send the signal that a "no ******" button is okay. Queer characters should be walking around just like queer people walk around in the real world. Even if the player chooses not to interact with them, not to romance them, not to go anywhere near them, they should not be able to press a button and make them all disappear.
The bisexual LIs of DA2 may effectively work the same, in that a straight character can be oblivious to their orientations and pretend they are straight. But they are still canonically bisexual. Their bisexuality does not disappear if you play as straight. They still exist.
Firstly, your username is fantastic.
Secondly, yeah I agree. Even if someone proposes a toggle with the best intentions, and only in the sense of "let's allow people to customise the game in the way they like"...
... it's still a terrible idea. The message it sends is that filtering same-sex characters from media is okay, that the developer is prepared to buckle under the pressure of the (frankly, homophobic) segment of the fanbase that wants a toggle to remove LGBT characters, and that the studio really thinks it should invest time and resources in deliberately removing content that people find objectionable. They do not do this with violence, slavery, racism, adult themes or horror.
It would be a PR disaster. They will not do it.
The perception of something is as important as the intention. Even if it were done with the best reasons, giving people a way to filter same-sex content can and would be seen as giving in to demands from immature people.