I believe a lot of it is a call to the silent majority. The bisexuals who have always been forced by the gay and straight communities to pick a side.
For too long, some gay and straight people have seen bisexuality as simply a gateway for closeted gay men to come out. This is true, but there is a much bigger number who are actually a shade of bisexual who enjoy both to varying degrees.
These shows are trying to make people aware that bisexuals can have relationships or even just sex with any gender and that it is normal for these people.
It's not erasure of gay identity. It's an acceptance that there are real bisexuals who never really identified as gay.