You also have to remember that very very few people know. Marten, Emily, Clinton, and Claire-Mom are the only people we KNOW in the comic actually know.
So, I'm a bisexual man. Yes, bisexuals do not face the same amount of discrimination, but we do face it. But I'm also married to a woman and have kids. I "pass" as a straight man, and most people don't even know I'm bi. I'm not secretive about it, but I don't generally talk about it unless something brings it up.
The area I live in is pretty blue (though not as blue as Northampton), but well... I don't tend to really face much discrimination. At worst I hear people occasionally say something that erases bisexuality, but I just respond to it and move on. Almost nothing is actually directed at me, because, well, most people just don't know.
I feel its pretty realistic for someone who is in a group that is discriminated against, with only a very select group of people they trust knowing, who lives in a massively liberal area, to not face much direct discrimination.
That being said, just the fact that I feel that hiding it leads to less trouble, is pretty shitty in and of itself. And I think about that in my own life. But... Its just easier not to be vocal about it if I don't need to. It sucks, but its practical.